{"id":11287,"date":"2026-09-27T14:52:13","date_gmt":"2026-09-27T13:52:13","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-1-molar-volume-of-a-gas\/"},"modified":"2026-10-03T08:38:11","modified_gmt":"2026-10-03T07:38:11","slug":"pag-1-molar-volume-of-a-gas","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-1-molar-volume-of-a-gas\/","title":{"rendered":"PAG 1 Moles Determination: Molar Volume of a Gas"},"content":{"rendered":"<script src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/three.js\/r128\/three.min.js\"><\/script>\n\n<section class=\"ols-revision-page ols-core-practical-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      --soft-green: #f0f7f1;\n      --soft-orange: #fff7ed;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      --inner-shadow: 0 10px 26px rgba(28, 36, 75, 0.08);\n      font-family: Poppins, Arial, sans-serif;\n      color: var(--navy);\n      background: #ffffff;\n    }\n\n    .ols-revision-page * { box-sizing: border-box; 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}\n    @media (hover: hover) and (pointer: fine) { .ols-zoom-card img.ols-zoomable-img:hover { transform: translateZ(0) scale(1.35) !important; box-shadow: 0 28px 70px rgba(28, 36, 75, 0.34) !important; filter: saturate(1.02) contrast(1.01) !important; z-index: 100 !important; } }\n    .ols-zoom-card-caption { padding: 18px 22px 20px !important; background: linear-gradient(135deg, #ffffff, #f8fbff) !important; border-bottom-left-radius: 26px !important; border-bottom-right-radius: 26px !important; position: relative !important; z-index: 1 !important; }\n    .ols-zoom-card-caption p { margin: 0 !important; color: #5f6b85 !important; font-size: 15px !important; line-height: 1.65 !important; font-weight: 300 !important; font-style: italic !important; font-family: Poppins, Arial, sans-serif !important; }\n    .ols-image-lightbox { position: fixed; inset: 0; z-index: 999999; display: none; align-items: center; justify-content: center; padding: 34px; background: rgba(10, 15, 35, 0.86); backdrop-filter: blur(8px); -webkit-backdrop-filter: blur(8px); }\n    .ols-image-lightbox.is-open { display: flex; }\n    .ols-image-lightbox-inner { position: relative; width: min(96vw, 1500px); max-height: 92vh; display: flex; align-items: center; justify-content: center; }\n    .ols-image-lightbox-img { display: block; max-width: 100%; max-height: 92vh; height: auto; width: auto; border-radius: 22px; background: #ffffff; box-shadow: 0 32px 90px rgba(0, 0, 0, 0.45); object-fit: contain; }\n    .ols-image-lightbox-close { position: absolute; top: -18px; right: -18px; width: 46px; height: 46px; border: 0; border-radius: 50%; background: #ffffff; color: var(--navy); font-family: Poppins, Arial, sans-serif; font-size: 28px; line-height: 1; font-weight: 700; cursor: pointer; box-shadow: 0 16px 34px rgba(0, 0, 0, 0.28); display: flex; align-items: center; justify-content: center; transition: transform 0.2s ease, background 0.2s ease, color 0.2s ease; }\n    .ols-image-lightbox-close:hover { transform: scale(1.08); background: var(--blue); color: #ffffff; }\n    @media (max-width: 760px) { .ols-zoom-card { border-radius: 22px !important; overflow: hidden !important; } .ols-zoom-card-image { min-height: auto !important; padding: 12px !important; overflow: hidden !important; border-top-left-radius: 22px !important; border-top-right-radius: 22px !important; } .ols-zoom-card img.ols-zoomable-img, .ols-zoom-card img.ols-zoomable-img:hover { transform: none !important; box-shadow: none !important; } .ols-zoom-card-caption { border-bottom-left-radius: 22px !important; border-bottom-right-radius: 22px !important; } .ols-image-lightbox { padding: 16px; } .ols-image-lightbox-inner { width: 100%; max-height: 88vh; } .ols-image-lightbox-img { max-height: 88vh; border-radius: 16px; } .ols-image-lightbox-close { top: 10px; right: 10px; width: 42px; height: 42px; font-size: 26px; } }\n\n\n    .ols-table-wrap { overflow: hidden; border-radius: 22px; border: 1px solid var(--border); background: #ffffff; margin-top: 18px; }\n    .ols-table { width: 100%; 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}\n      .ols-course-cta-card { padding: 20px; border-radius: 24px; }\n      .ols-course-cta-header-row { align-items: stretch; }\n      .ols-course-button-top, .ols-course-button { width: 100%; }\n    }\n  \n    .ols-figure-placeholder .ols-placeholder-box { border: 2px dashed #c9973a; background: #fffaf0; border-radius: 16px; padding: 26px 22px; font-weight: 700; color: #7a4b12; text-align: center; line-height: 1.6; }\n    .ols-figure-placeholder .ols-figure-image { background: transparent; }\n    .ols-figure-placeholder { background: #ffffff; border: 1px solid rgba(28, 36, 75, 0.12); border-radius: 22px; padding: 14px; margin: 18px 0 6px; }\n    .ols-figure-placeholder .ols-figure-caption p { margin: 10px 0 0; font-size: 14px; color: #667085; text-align: center; }\n\n    \/* inline checks (H5P re-flow, Sep 2026) *\/\n    .ols-h5p-card.ols-h5p-inline { padding: 26px 28px; border-left: 6px solid #7c3aed; }\n    .ols-h5p-card.ols-h5p-inline h2 { font-size: clamp(20px, 2.2vw, 27px); letter-spacing: -0.02em; }\n    .ols-h5p-card.ols-h5p-inline > p { margin: 8px 0 0; }\n    .ols-h5p-card.ols-h5p-inline .ols-h5p-frame { margin-top: 16px; padding: 14px; border-radius: 20px; }\n    .ols-h5p-kicker { display: inline-block; margin-bottom: 10px; padding: 5px 12px; border-radius: 999px; background: #ede9fe; color: #5b21b6; font-size: 12px; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; }\n    .ols-h5p-card.ols-h5p-recap { border-left-color: #c9973a; background: linear-gradient(135deg, #ffffff 0%, #fff8e8 100%); }\n    .ols-h5p-recap .ols-h5p-kicker { background: #fdf0d2; color: #8a5a00; }\n    @media (max-width: 760px) { .ols-h5p-card.ols-h5p-inline { padding: 20px 16px; } }\n\n    \/* core practical pages (Sep 2026) *\/\n    .ols-note-card h3 { margin: 22px 0 8px; font-size: 18px; line-height: 1.25; font-weight: 500; color: var(--navy); }\n    .ols-table td { overflow-wrap: anywhere; }\n<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>OCR A A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Practical Activity Groups (PAGs)<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-1-molar-volume-of-a-gas\/\">PAG 1: Molar Volume of a Gas<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-3-enthalpy-determination\/\">PAG 3: Enthalpy Change via Hess's Law<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-acid-base-titration\/\">PAG 2: Concentration of HCl by Titration<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-preparing-a-standard-solution\/\">PAG 2: Preparation of a Standard Solution<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/rates-of-hydrolysis-of-haloalkanes\/\">Hydrolysis rates: Hydrolysis of Halogenoalkanes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-5-synthesis-of-an-organic-liquid\/\">PAG 5: Chlorination of 2-methylpropan-2-ol<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-5-oxidation-of-an-alcohol\/\">PAG 5: Oxidation of an Alcohol<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-4-pag-7-qualitative-analysis-of-ions-and-functional-groups\/\">PAG 4 and 7: Analysis of Inorganic and Organic Unknowns<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/measuring-rates-of-reaction\/\">Rates: Rates of Reaction<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-1-moles-determination-by-mass\/\">PAG 1: Gravimetric and Composition Analysis<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Useful Links<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/\">OCR A Chemistry<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/\">Practical Activity Groups (PAGs)<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\ndocument.addEventListener('DOMContentLoaded', function () {\n  var currentPath = window.location.pathname.replace(\/\\\/$\/, '');\n  var sidebarLinks = document.querySelectorAll('.ols-sidebar .ols-topic-list a');\n  sidebarLinks.forEach(function (link) {\n    var linkPath = new URL(link.href, window.location.origin).pathname.replace(\/\\\/$\/, '');\n    if (linkPath === currentPath) { link.closest('li').classList.add('active'); } else { link.closest('li').classList.remove('active'); }\n  });\n});\n<\/script>\n\n  <main class=\"ols-main\">\n      <nav class=\"ols-breadcrumbs\" aria-label=\"Breadcrumb\">\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/\">OCR A<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/\">Practical Activity Groups (PAGs)<\/a> \/\n<span>PAG 1 Moles Determination: Molar Volume of a Gas<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>PAG 1 Moles Determination: Molar Volume of a Gas<\/h1>\n        <p class=\"ols-page-intro\">PAG 1 (OCR A A Level Chemistry): measuring the molar volume of carbon dioxide from calcium carbonate and ethanoic acid, with the apparatus and its reasons, weighing by difference, the volume against mass graph, the full mole calculation, the excess check, and the errors, uncertainties and improvements examiners ask about.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Practical endorsement<\/div>\n<div class=\"ols-badge\">PAG 1<\/div>\n<div class=\"ols-badge\">H432<\/div>\n<\/div>\n\n        <div class=\"ols-author\">\n\n    <img decoding=\"async\"\n      class=\"ols-author-avatar-img\"\n      src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n      alt=\"Dr. Mohammed Al-Fatah\"\n    >\n\n    <div class=\"ols-author-content\">\n\n      <h2 class=\"ols-author-title\">\n        Written by:<br><span>Dr. Mohammed Al-Fatah<\/span>\n      <\/h2>\n\n      <p class=\"ols-author-description\">Chemistry specialist revision notes for OCR A A Level Chemistry.<\/p>\n\n      <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n      <\/header>\n\n      <section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>GCSE Recap: Moles, Gas Collection and 24 dm\u00b3<\/h2>\n<p>Three quick questions on the GCSE ideas this practical builds on: moles from mass, which gases can be collected over water, and the molar gas volume.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"977\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>What This Practical Is Testing<\/h2>\n<\/div>\n<p>This practical, PAG 1 on the OCR A course, measures the <strong>molar volume<\/strong> of a gas: the volume occupied by one mole of carbon dioxide at room temperature and pressure.<\/p><p>The value depends on both temperature and pressure, so both are recorded and quoted with the answer.<\/p><p>Carbon dioxide is made by reacting weighed masses of calcium carbonate with an excess of ethanoic acid. The gas is collected over water in a 100 cm\u00b3 measuring cylinder, and the volume is plotted against the mass of solid used.<\/p>\n<p class=\"ols-equation\">CaCO\u2083(s) + 2CH\u2083COOH(aq) \u2192 (CH\u2083COO)\u2082Ca(aq) + CO\u2082(g) + H\u2082O(l)<\/p>\n<p>The equation gives a <strong>1 : 1 ratio<\/strong> between calcium carbonate and carbon dioxide, so the amount of gas in moles equals the amount of solid in moles. Molar volume is simply the volume of gas divided by that amount.<\/p>\n<p>Because the acid is in excess, the volume of gas depends only on the mass of calcium carbonate, which is why the graph is a straight line through the origin.<\/p>\n<p>On the OCR A course this practical is examined through the written papers (practical skills are examined in every paper) and the practical endorsement.<\/p><p>Questions ask you to justify the apparatus, explain why the solid is weighed before and after tipping, read a value from a line of best fit, and complete the mole calculation with units and significant figures.<\/p><p>They also ask you to state the direction of each error, and suggest a gas syringe or a larger mass as an improvement.<\/p><p>The same mole ideas are tested in <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-preparing-a-standard-solution\/\">PAG 2 (preparation of a standard solution)<\/a> and <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-acid-base-titration\/\">PAG 2 (concentration of hcl by titration)<\/a>.<\/p>\n\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Molar volume = volume of gas \u00f7 amount of gas in moles. Quote it with its conditions: about 24 dm\u00b3 mol\u207b\u00b9 at room temperature and pressure.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Measuring Gas Volumes<\/h2>\n<\/div>\n<p>There are two ways to measure a gas volume in the laboratory. A <strong>gas syringe<\/strong> collects the gas directly: the plunger is pushed out as gas enters and the volume is read from the barrel.<\/p><p><strong>Collecting over water<\/strong> uses a measuring cylinder that is filled with water, inverted in a trough and clamped vertically.<\/p><p>Gas bubbling in through a delivery tube collects at the closed end and pushes the water level down, and the volume is read from the graduations at the water level.<\/p><p>This practical uses a <strong>100 cm\u00b3 measuring cylinder<\/strong> over water, and its capacity sets the largest mass of solid that can be used.<\/p>\n<p>Collecting over water only works for gases that are almost insoluble and do not react with water. Hydrogen, oxygen and nitrogen are fine; ammonia, hydrogen chloride and sulfur dioxide are not.<\/p><p>Carbon dioxide is <strong>slightly soluble<\/strong>, so a little of it dissolves in the trough and the measured volume is slightly too low.<\/p><p>Working in the other direction, any gas collected over water is <strong>saturated with water vapour<\/strong>, which adds to the volume and makes the reading slightly too high.<\/p><div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> The two effects partly cancel, but an examiner expects you to name both and give their directions.<\/p><\/div>\n<p>Three rules for the reading itself. Wait until the gas has returned to <strong>room temperature<\/strong>, because the reaction is exothermic and warm gas occupies more space. Keep the cylinder <strong>vertical<\/strong> and read at <strong>eye level<\/strong> to avoid parallax.<\/p><p>There is no need to level the water inside and outside the cylinder, because it is the volume of the gas space, not its pressure, that is being read.<\/p><p>The small pressure difference is one reason the answer is quoted only to two significant figures.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-molar-methods.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-molar-methods.jpg\" alt=\"Gas syringe beside collection over water in an inverted 100 cm\u00b3 measuring cylinder, with the errors of each method\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Gas syringe and collection over water side by side, with the errors of each method and the reminder to record room temperature and pressure.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Carbon dioxide is slightly soluble in water, so some is lost and the volume collected is too low; a gas syringe avoids this.&#8221; Name the gas, the direction and the fix.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Which Gases Can Be Collected Over Water<\/h2>\n<p>Decide which gases suit which method and which way each error pushes the reading.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-978\" class=\"h5p-iframe\" data-content-id=\"978\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Molar Volume of a Gas Drag: Which Gases Can Be Collected Over Water\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Safety and Apparatus<\/h2>\n<\/div>\n<p>Every item has a job and, where it is used for a measurement, a precision that you should be able to quote. The balance and the measuring cylinder are the two instruments whose uncertainties carry into the result; the acid volume does not matter because the acid is in excess.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Apparatus<\/th><th>What it is for<\/th><th>Precision<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Boiling tube, rubber bung and delivery tube<\/strong><\/td><td>Reaction vessel for 30 cm\u00b3 of acid; the bung seals it so that every bubble of gas travels down the tube<\/td><td>Bung must be airtight and fitted immediately<\/td><\/tr>\n<tr><td><strong>100 cm\u00b3 measuring cylinder<\/strong><\/td><td>Inverted over water to collect and measure the carbon dioxide<\/td><td>Graduated in 1 cm\u00b3, read to \u00b10.5 cm\u00b3<\/td><\/tr>\n<tr><td><strong>Water trough and two clamp stands<\/strong><\/td><td>Holds the water seal; one clamp holds the boiling tube, the other keeps the cylinder vertical<\/td><td>Cylinder must be vertical for a true reading<\/td><\/tr>\n<tr><td><strong>Balance reading to 2 decimal places<\/strong><\/td><td>Weighs the test tube and calcium carbonate before and after tipping<\/td><td>\u00b10.005 g per reading, \u00b10.01 g for the difference<\/td><\/tr>\n<tr><td><strong>Test tube and spatula<\/strong><\/td><td>Holds the weighed calcium carbonate powder; weighed again after tipping so the mass used is known exactly<\/td><td>Any powder left behind is not counted<\/td><\/tr>\n<tr><td><strong>50 cm\u00b3 measuring cylinder<\/strong><\/td><td>Measures 30 cm\u00b3 of 1 mol dm\u207b\u00b3 ethanoic acid<\/td><td>\u00b10.5 cm\u00b3, not critical because the acid is in excess<\/td><\/tr>\n<tr><td><strong>Thermometer and barometer (or weather data)<\/strong><\/td><td>Records room temperature and atmospheric pressure so the molar volume can be quoted at stated conditions<\/td><td>\u00b10.5 \u00b0C; pressure to the nearest 1 kPa<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Once the bung is fitted the whole train of apparatus is a closed system with one outlet: the end of the delivery tube under the mouth of the cylinder.<\/p><p>Gas produced in the boiling tube pushes the air already in the tube ahead of it, so the <strong>first bubbles are air<\/strong>.<\/p><p>Keep them: at the end of the run an equal volume of carbon dioxide is left behind in the tube and the space above the acid, so the two exactly cancel and the volume in the cylinder is the volume of carbon dioxide made.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-molar-apparatus.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-molar-apparatus.jpg\" alt=\"The assembled apparatus: clamped boiling tube of ethanoic acid, bung and delivery tube, trough, inverted 100 cm\u00b3 measuring cylinder with upright graduations, thermometer\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The assembled apparatus: clamped boiling tube of ethanoic acid, airtight bung and delivery tube, water trough and inverted 100 cm\u00b3 measuring cylinder read at the water level.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Safety:<\/strong> 1 mol dm\u207b\u00b3 ethanoic acid is an irritant: wear eye protection and rinse splashes at once. Calcium carbonate is low hazard.<\/p><p>Never let the delivery tube become blocked or pinched while gas is being produced: pressure builds and the bung can be ejected. Clamp the boiling tube so it cannot tip when the powder is added.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Method: Step by Step<\/h2>\n<\/div>\n<p>The method is built around two ideas. The first is knowing exactly how much solid reacted, which is why the test tube is weighed <strong>before and after<\/strong> tipping.<\/p>\n<p>The second is losing no gas, which is why the bung goes on the instant the powder is in.<\/p>\n<p>Every step below carries its reason, because the reason is what the mark scheme rewards.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Step<\/th><th>What you do<\/th><th>Why<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1<\/strong><\/td><td>Fill the 100 cm\u00b3 measuring cylinder with water, invert it in the trough without letting air in, clamp it vertically and place the end of the delivery tube under its mouth.<\/td><td>The cylinder must start full of water so that every bubble collected displaces water and is measured. Air at the top before you start would be counted as gas.<\/td><\/tr>\n<tr><td><strong>2<\/strong><\/td><td>Measure 30 cm\u00b3 of 1 mol dm\u207b\u00b3 ethanoic acid into the boiling tube and clamp it. Fit the bung and delivery tube loosely ready for use.<\/td><td>The acid is in large excess (checked below), so its volume does not need high precision. Having the bung ready means it can be fitted in under a second.<\/td><\/tr>\n<tr><td><strong>3<\/strong><\/td><td>Put about 0.05 g of calcium carbonate powder in a dry test tube. Weigh the test tube and contents on a balance reading to 2 decimal places (0.01 g) and record the mass.<\/td><td>The mass used is found by difference, so the reading before tipping is one of the two numbers you need. Powder reacts faster than chips, so the run finishes quickly.<\/td><\/tr>\n<tr><td><strong>4<\/strong><\/td><td>Tip the powder into the acid and fit the bung immediately.<\/td><td>The reaction is fastest in the first seconds. Any gas made before the bung is in escapes to the room and the volume is too low, a systematic error that cannot be corrected for.<\/td><\/tr>\n<tr><td><strong>5<\/strong><\/td><td>Reweigh the empty test tube while the gas is collecting. The difference between the two readings is the mass of calcium carbonate that reacted.<\/td><td>Some powder always clings to the tube. Weighing by difference counts only the solid that went in, which is what the mole calculation needs.<\/td><\/tr>\n<tr><td><strong>6<\/strong><\/td><td>When no more bubbles appear and the water level stops moving, allow the gas to return to room temperature, then read the volume at the water level with the cylinder vertical and your eye level with the meniscus.<\/td><td>The reaction is exothermic: warm gas occupies more volume, so an early reading is too high. Reading off-vertical or above eye level introduces parallax.<\/td><\/tr>\n<tr><td><strong>7<\/strong><\/td><td>Record room temperature and atmospheric pressure.<\/td><td>The molar volume is only meaningful at stated conditions; the value at 20 \u00b0C and 101 kPa differs from the value at 25 \u00b0C and 100 kPa.<\/td><\/tr>\n<tr><td><strong>8<\/strong><\/td><td>Repeat with fresh acid and masses increasing by about 0.05 g each time up to 0.40 g, refilling the cylinder with water between runs.<\/td><td>Several masses give a line of best fit, which averages random error and shows up an anomalous run. Fresh acid keeps the excess large in every run.<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-molar-weighing.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-molar-weighing.jpg\" alt=\"Weighing by difference: test tube with powder on a 2 d.p. balance before and after tipping into the acid\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Weighing by difference: the test tube is weighed with the powder (14.87 g), the powder is tipped into the acid, and the tube is reweighed (14.82 g), so 0.05 g reacted.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Why stop at 0.40 g:<\/strong> 0.40 g of CaCO\u2083 is 0.40 \u00f7 100.1 = 0.0040 mol, which would give about 0.0040 \u00d7 24 000 = 96 cm\u00b3 of CO\u2082, almost filling the 100 cm\u00b3 cylinder. A larger mass would push gas out of the mouth of the cylinder and the run would be lost.<\/p>\n<\/div>\n<\/article>\n<!-- 3D card: molar-volume-co2 (27 Sep 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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height:32px;}\n\n@media (max-width:760px){\n  .ols-mvc-001{padding:26px; border-radius:22px;}\n  .ols-mvc-001 h2.mvc-title{font-size:20.5px;}\n  .ols-mvc-001 p.mvc-sub{font-size:13.5px;}\n  .ols-mvc-001 .mvc-stage{height:560px;}\n}\n@media (max-width:560px){\n  .ols-mvc-001 .mvc-player{padding:6px 8px;}\n  .ols-mvc-001 .mvc-prog{order:-1; flex:1 1 100%; margin:0 4px;}\n  .ols-mvc-001 .mvc-pb{width:36px; height:36px;}\n  .ols-mvc-001 .mvc-time{margin-right:auto;}\n  .ols-mvc-001 .mvc-chip{font-size:11.5px; padding:5px 10px;}\n}\n@media (max-width:480px){\n  .ols-mvc-001{padding:14px; border-radius:18px;}\n  .ols-mvc-001 .mvc-head{margin-bottom:14px;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-mvc-001 .mvc-poster,.ols-mvc-001 .mvc-chip,.ols-mvc-001 .mvc-pb{transition:none;}\n}\n\n\/* full screen: the card fills the screen and the stage takes the spare height *\/\n.ols-mvc-001:fullscreen{max-width:none; width:100%; height:100%; margin:0; border-radius:0; border:0; padding:16px 20px; display:flex; flex-direction:column; overflow:auto;}\n.ols-mvc-001:fullscreen .mvc-head{display:none;}\n.ols-mvc-001:fullscreen .mvc-stage{flex:1 1 auto; height:auto; min-height:320px;}\n.ols-mvc-001:-webkit-full-screen{max-width:none; width:100%; height:100%; margin:0; border-radius:0; border:0; padding:16px 20px; display:flex; flex-direction:column; overflow:auto;}\n.ols-mvc-001:-webkit-full-screen .mvc-head{display:none;}\n.ols-mvc-001:-webkit-full-screen .mvc-stage{flex:1 1 auto; height:auto; min-height:320px;}\n\n.ols-cc-mvc-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  margin:10px auto 0; text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-mvc-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-mvc-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"mvc-head\">\n  <h2 class=\"mvc-title\">Molar Volume of CO<sub>2<\/sub> Bench<\/h2>\n  <p class=\"mvc-sub\">Collect the carbon dioxide made when calcium carbonate reacts with ethanoic acid, repeat with different masses, and use a graph to find the volume of one mole of gas at room temperature and pressure.<\/p>\n<\/div>\n\n<div class=\"mvc-stage\" id=\"mvcStage\" tabindex=\"0\" aria-label=\"Film of the practical. Space plays or pauses, the left and right arrow keys skip 10 seconds.\">\n  <canvas class=\"mvc-canvas\" id=\"mvcCanvas\" aria-hidden=\"true\"><\/canvas>\n  <div class=\"mvc-overlay\" id=\"mvcOverlay\"><\/div>\n  <div class=\"mvc-subt\" id=\"mvcSubt\"><\/div>\n  <div class=\"mvc-hint\" id=\"mvcHint\" hidden>Paused: drag to look around<\/div>\n  <button type=\"button\" class=\"mvc-poster\" id=\"mvcPoster\" aria-label=\"Play the film\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M7 4.5v15l12.5-7.5z\"\/><\/svg><span>Replay<\/span><\/button>\n<\/div>\n\n<div class=\"mvc-player\" id=\"mvcPlayer\" role=\"group\" aria-label=\"Film controls\">\n  <button type=\"button\" class=\"mvc-pb\" id=\"mvcPlay\" aria-label=\"Play\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M7 4.5v15l12.5-7.5z\"\/><\/svg><\/button>\n  <button type=\"button\" class=\"mvc-pb\" id=\"mvcBack\" aria-label=\"Back 10 seconds\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M12 4V1.5L7.5 5 12 8.5V6a7 7 0 1 1-7 7H3a9 9 0 1 0 9-9z\"\/><text x=\"12.2\" y=\"16.2\" text-anchor=\"middle\">10<\/text><\/svg><\/button>\n  <button type=\"button\" class=\"mvc-pb\" id=\"mvcFwd\" aria-label=\"Forward 10 seconds\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M12 4V1.5L16.5 5 12 8.5V6a7 7 0 1 0 7 7h2a9 9 0 1 1-9-9z\"\/><text x=\"11.8\" y=\"16.2\" text-anchor=\"middle\">10<\/text><\/svg><\/button>\n  <button type=\"button\" class=\"mvc-pb\" id=\"mvcRestart\" aria-label=\"Restart\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M5.5 5h2.2v14H5.5zM9.2 12l9.3 6.8V5.2z\"\/><\/svg><\/button>\n  <div class=\"mvc-prog\" id=\"mvcProg\" role=\"slider\" tabindex=\"0\" aria-label=\"Seek\" aria-valuemin=\"0\" aria-valuemax=\"199\" aria-valuenow=\"0\" aria-valuetext=\"0:00\">\n    <div class=\"mvc-track\"><div class=\"mvc-fill\" id=\"mvcFill\"><\/div><\/div>\n    <div class=\"mvc-thumb\" id=\"mvcThumb\"><\/div>\n  <\/div>\n  <span class=\"mvc-time\" id=\"mvcTime\">0:00 \/ 3:19<\/span>\n  <button type=\"button\" class=\"mvc-pb mvc-speed\" id=\"mvcSpeed\" aria-label=\"Playback speed 1\u00d7\">1\u00d7<\/button>\n  <button type=\"button\" class=\"mvc-pb\" id=\"mvcFs\" aria-label=\"Full screen\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M4 9V4h5v2H6v3zm11-5h5v5h-2V6h-3zM4 15h2v3h3v2H4zm14 3v-3h2v5h-5v-2z\"\/><\/svg><\/button>\n<\/div>\n<div class=\"mvc-chips\" id=\"mvcChips\" role=\"group\" aria-label=\"Chapters\"><\/div>\n<\/section>\n\n<p class=\"ols-cc-mvc-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/molar-volume-co2.js?v=20260928b\"><\/script>\n\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: The Order of the Method<\/h2>\n<p>Put the steps of a hydrogen version of this practical in a workable order.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-979\" class=\"h5p-iframe\" data-content-id=\"979\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Molar Volume of a Gas Order: The Hydrogen Version of the Method\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Results and the Graph<\/h2>\n<\/div>\n<p>Seven runs give the results below. Experimental data are <strong>scattered by random error<\/strong>: small differences in how quickly the bung went on, how long the gas was left to cool and how the water level was read.<\/p><p>A single run could therefore be badly wrong, and a mean of seven runs at different masses is meaningless. Instead the results are plotted and a <strong>line of best fit<\/strong> is drawn.<\/p><p>The line averages out the random errors, and a point that lies well off it can be identified as anomalous and ignored.<\/p><p>That is why the practical uses several masses rather than one, and why the final volume is read from the line rather than from any single run.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Mass of CaCO\u2083 \/ g<\/th><th>Volume of CO\u2082 collected \/ cm\u00b3<\/th><th>Volume predicted by the line \/ cm\u00b3<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>0.05<\/strong><\/td><td>11<\/td><td>12<\/td><\/tr>\n<tr><td><strong>0.11<\/strong><\/td><td>27<\/td><td>26<\/td><\/tr>\n<tr><td><strong>0.17<\/strong><\/td><td>32<\/td><td>41<\/td><\/tr>\n<tr><td><strong>0.21<\/strong><\/td><td>50<\/td><td>50<\/td><\/tr>\n<tr><td><strong>0.24<\/strong><\/td><td>59<\/td><td>58<\/td><\/tr>\n<tr><td><strong>0.32<\/strong><\/td><td>74<\/td><td>77<\/td><\/tr>\n<tr><td><strong>0.33<\/strong><\/td><td>80<\/td><td>79<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Mass of calcium carbonate is the <strong>independent variable<\/strong> and goes on the x-axis; volume of carbon dioxide is the <strong>dependent variable<\/strong> and goes on the y-axis.<\/p><p>The line of best fit is a straight line through the origin, because no solid reacting means no gas produced, and it is positioned so that the points are scattered evenly either side of it.<\/p><p>The third column shows what the line predicts at each mass: six runs lie within 3 cm\u00b3 of the line, while the 0.17 g run gave 32 cm\u00b3 against a predicted 41 cm\u00b3 and is <strong>anomalous<\/strong>.<\/p><p>This is most likely because gas escaped before the bung was fitted. It is ignored when the line is drawn.<\/p><p>The gradient of the line is about 240 cm\u00b3 g\u207b\u00b9, and reading it at 0.25 g gives about 60 cm\u00b3.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/wave-8.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/wave-8.jpg\" alt=\"Molar volume of carbon dioxide: results table, graph read off at 0.25 g giving 60 cm\u00b3, and the six-step calculation to 24 dm\u00b3 mol\u207b\u00b9\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The results table, the graph with its line of best fit through the origin read at 0.25 g to give 60 cm\u00b3, and the calculation to 24 dm\u00b3 mol\u207b\u00b9.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Graph rule:<\/strong> Independent variable on the x-axis, dependent on the y-axis, units in the axis labels as &#8220;mass of CaCO\u2083 \/ g&#8221; and &#8220;volume of CO\u2082 \/ cm\u00b3&#8221;, a straight line through the origin with equal scatter either side, and any anomaly circled and labelled.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Calculating the Molar Volume<\/h2>\n<\/div>\n<p>The graph is read at <strong>0.25 g<\/strong>, a mass inside the measured range that does not correspond to any single run, so the value comes from the line, not from one measurement.<\/p>\n<p>Every step is written out with its unit; students lose marks by jumping from 60 cm\u00b3 to 24 dm\u00b3 mol\u207b\u00b9 without showing the moles.<\/p>\n<p class=\"ols-equation\">amount of CaCO\u2083 = mass \u00f7 molar mass = 0.25 g \u00f7 100.1 g mol\u207b\u00b9 = 0.00250 mol<\/p>\n<p class=\"ols-equation\">amount of CO\u2082 = 0.00250 mol (1 : 1 ratio in the equation)<\/p>\n<p class=\"ols-equation\">molar volume = volume \u00f7 amount = 60 cm\u00b3 \u00f7 0.00250 mol = 24 000 cm\u00b3 mol\u207b\u00b9 = 24 dm\u00b3 mol\u207b\u00b9<\/p>\n<p>The molar mass of CaCO\u2083 is 40.1 + 12.0 + (3 \u00d7 16.0) = 100.1 g mol\u207b\u00b9. The volume was read from the graph to <strong>2 significant figures<\/strong>, so the answer is quoted to 2 significant figures: 24 dm\u00b3 mol\u207b\u00b9 at room temperature and pressure, taken as 293 K and 101 kPa. Writing 24.0 dm\u00b3 mol\u207b\u00b9 claims a precision the reading does not have.<\/p>\n<p>A second route that examiners like uses the <strong>gradient<\/strong> directly. The gradient of the line is the volume of gas per gram of solid, so multiplying by the mass of one mole gives the volume of one mole.<\/p><p>Molar volume = gradient \u00d7 Mr = 240 cm\u00b3 g\u207b\u00b9 \u00d7 100.1 g mol\u207b\u00b9 = 24 000 cm\u00b3 mol\u207b\u00b9.<\/p><p>The two routes must agree because reading at 0.25 g is just the gradient multiplied by 0.25.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Show the units:<\/strong> g \u00f7 g mol\u207b\u00b9 = mol, then cm\u00b3 \u00f7 mol = cm\u00b3 mol\u207b\u00b9, then \u00f7 1000 for dm\u00b3 mol\u207b\u00b9. A unit chain that works is proof the method is right.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">7<\/div>\n<h2>Comparing With the Accepted Value<\/h2>\n<\/div>\n<p>The accepted molar volume of an ideal gas at room temperature and pressure is <strong>24.0 dm\u00b3 mol\u207b\u00b9<\/strong>.<\/p><p>A result of 24 dm\u00b3 mol\u207b\u00b9 agrees with it within the precision of the reading, and the comment &#8220;the result agrees with the accepted value to 2 significant figures&#8221; earns the evaluation mark.<\/p><p>A stricter reading of the line at 0.25 g gives 59 cm\u00b3 rather than 60 cm\u00b3, and then:<\/p>\n<p class=\"ols-equation\">molar volume = 59 \u00f7 0.00250 = 23 600 cm\u00b3 mol\u207b\u00b9 = 23.6 dm\u00b3 mol\u207b\u00b9<\/p>\n<p class=\"ols-equation\">percentage error = (23.6 \u2212 24.0) \u00f7 24.0 \u00d7 100 = \u22121.7%<\/p>\n<p>The negative sign matters: it says the experiment gives a molar volume that is <strong>too low<\/strong>.<\/p>\n<p>That is exactly what the two main systematic errors, gas escaping before the bung is fitted and carbon dioxide dissolving in the water, would produce.<\/p>\n<p>A percentage error whose sign matches the expected direction of the systematic errors is strong evidence that the method, not carelessness, is responsible.<\/p>\n<p>The accepted value itself depends on conditions, which is why they are recorded.<\/p><p>The ideal gas equation pV = nRT gives, for one mole at the recorded 21 \u00b0C (294 K) and 101 kPa, V = nRT \u00f7 p = 1 \u00d7 8.314 \u00d7 294 \u00f7 101 000 = 0.0242 m\u00b3 = 24.2 dm\u00b3.<\/p><p>Comparing 24 dm\u00b3 mol\u207b\u00b9 with 24.2 dm\u00b3 mol\u207b\u00b9 is the fairest test of the experiment, and quoting the conditions with the result is what allows anyone else to make the same comparison.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;The result is lower than the accepted value. Gas escaped before the bung was fitted and some CO\u2082 dissolved in the water, both of which make the volume collected, and so the molar volume, too low.&#8221;<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: The Calculation With New Data<\/h2>\n<p>Carry out the full calculation, both routes, on a magnesium carbonate experiment and comment on the error.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"980\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">8<\/div>\n<h2>Checking That Ethanoic Acid Is in Excess<\/h2>\n<\/div>\n<p>For the volume of gas to depend only on the mass of solid, calcium carbonate must be the <strong>limiting reagent<\/strong> in every run, so the acid must be in excess even for the largest mass allowed. This check should appear in your write-up and is a common two-mark question.<\/p>\n<p class=\"ols-equation\">largest mass of CaCO\u2083 = 0.40 g, so amount = 0.40 \u00f7 100.1 = 0.003996 mol<\/p>\n<p class=\"ols-equation\">amount of CH\u2083COOH available = 1.00 mol dm\u207b\u00b3 \u00d7 30 \u00f7 1000 dm\u00b3 = 0.030 mol<\/p>\n<p class=\"ols-equation\">amount of CH\u2083COOH needed = 2 \u00d7 0.003996 = 0.00799 mol<\/p>\n<p>The acid available, 0.030 mol, is nearly four times the 0.00799 mol needed, so ethanoic acid is in excess and calcium carbonate is the limiting reagent in every run.<\/p><p>That is why the volume of carbon dioxide is proportional to the mass of calcium carbonate and the graph is a straight line through the origin.<\/p>\n<p>The excess is large for one run but not for all of them together. The seven masses add up to 1.43 g, or 0.0143 mol of calcium carbonate, which would need 2 \u00d7 0.0143 = 0.0286 mol of acid.<\/p>\n<p>If the same 30 cm\u00b3 of acid were reused for every run the acid would still be in excess, but only just, and the last runs would be reacting in a much weaker solution and finishing slowly.<\/p>\n<div class=\"ols-key-box\"><p><strong>Exam tip:<\/strong> Using fresh acid for each run costs nothing and keeps the excess unquestionable; if you are told the acid was reused, say that the excess is marginal and the largest masses are the least reliable.<\/p><\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Moles of acid available (0.030 mol) is greater than moles needed (2 \u00d7 0.0040 = 0.0080 mol), so the acid is in excess and CaCO\u2083 is the limiting reagent.&#8221; Show both numbers and the comparison.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Limiting Reagent<\/h2>\n<p>Decide whether the acid is in excess for a different carbonate and a different acid.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-981\" class=\"h5p-iframe\" data-content-id=\"981\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Molar Volume of a Gas Fill In: Is the Acid in Excess?\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">9<\/div>\n<h2>Errors, Uncertainty and Improvements<\/h2>\n<\/div>\n<p>Evaluation questions ask for the source of an error, its <strong>direction<\/strong> (does it make the molar volume too high or too low?) and an improvement that removes it.<\/p><p>Random errors scatter the points and are dealt with by the line of best fit; systematic errors shift every point the same way and can only be dealt with by changing the method.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Source of error<\/th><th>Effect on the result<\/th><th>Improvement<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Gas escapes before the bung is fitted<\/strong><\/td><td>Volume too low, so molar volume too low; systematic, and cannot be corrected for<\/td><td>Have the bung in hand, drop the solid in and seal in one movement; or suspend the solid in a small tube inside the flask and tip it after sealing<\/td><\/tr>\n<tr><td><strong>Carbon dioxide dissolves in the water in the trough<\/strong><\/td><td>Volume too low, so molar volume too low; systematic<\/td><td>Collect the gas in a gas syringe instead of over water<\/td><\/tr>\n<tr><td><strong>Gas collected over water is saturated with water vapour<\/strong><\/td><td>Volume slightly too high, so molar volume slightly too high; systematic<\/td><td>Use a gas syringe, or correct for the vapour pressure of water at the recorded temperature<\/td><\/tr>\n<tr><td><strong>Some carbon dioxide stays dissolved in the acid solution<\/strong><\/td><td>Volume too low, so molar volume too low<\/td><td>Swirl the tube at the end of the run to release dissolved gas before reading<\/td><\/tr>\n<tr><td><strong>Gas is warm from the exothermic reaction and read at once<\/strong><\/td><td>Volume too high until it cools<\/td><td>Wait until the water level stops moving and the tube is at room temperature before reading<\/td><\/tr>\n<tr><td><strong>Cylinder not vertical or read above eye level (parallax)<\/strong><\/td><td>Random error in either direction<\/td><td>Clamp the cylinder vertical and read with the eye level with the water surface<\/td><\/tr>\n<tr><td><strong>Two balance readings of \u00b10.005 g on a 0.05 g sample<\/strong><\/td><td>\u00b10.01 g on 0.05 g is a 20% uncertainty; the smallest masses carry the largest percentage error<\/td><td>Use a balance reading to 3 decimal places, use larger masses, and take the value from the line of best fit rather than a single run<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-molar-errors.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/core-practical-molar-errors.jpg\" alt=\"Five error cards with arrows showing whether each makes the molar volume too high, too low or leaves it unchanged\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Direction of each error: gas escaping and dissolved CO\u2082 make the molar volume too low, water vapour and warm gas make it too high, and the first bubbles of displaced air cause no error at all.<\/p><\/div>\n<\/div>\n<p>Percentage uncertainty is (uncertainty \u00f7 reading) \u00d7 100, using the actual uncertainty of the instrument. The balance reads to \u00b10.005 g, and a mass by difference uses two readings, so its uncertainty is \u00b10.01 g. The 100 cm\u00b3 cylinder is read to \u00b10.5 cm\u00b3. For the smallest run, 0.05 g and 11 cm\u00b3:<\/p>\n<p class=\"ols-equation\">mass: 0.01 \u00f7 0.05 \u00d7 100 = 20%; volume: 0.5 \u00f7 11 \u00d7 100 = 4.5%; total = 24.5%<\/p>\n<p>For the largest run, 0.33 g and 80 cm\u00b3, the same working gives 0.01 \u00f7 0.33 \u00d7 100 = 3.0% and 0.5 \u00f7 80 \u00d7 100 = 0.6%, a total of 3.6%.<\/p><p>The mass, not the volume, dominates the uncertainty at every mass, so the useful improvements are a 3 decimal place balance (\u00b10.0005 g, cutting the mass uncertainty tenfold) and larger masses, not a finer measuring cylinder.<\/p><p>The graph is itself an improvement: reading from the line at 0.25 g uses all seven runs, so the random part of these uncertainties is largely averaged out.<\/p>\n<p>When the question asks for the &#8220;absolute uncertainty&#8221; of the mass, quote \u00b10.01 g (two readings of \u00b10.005 g); when it asks for the percentage uncertainty of the final molar volume, add the percentage uncertainties of the mass and the volume for the run used and quote the total to 2 significant figures.<\/p>\n\n<div class=\"ols-key-box\">\n<p><strong>Random or systematic:<\/strong> A slow bung, dissolved CO\u2082 and water vapour shift every run the same way (systematic): the line of best fit cannot remove them. Parallax and timing scatter the points (random): the line does remove them.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Which Way Does Each Error Push the Result<\/h2>\n<p>Predict the direction of each error in experiments on hydrogen and oxygen.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"982\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">10<\/div>\n<h2>Variant: Hydrogen From Magnesium in a Gas Syringe<\/h2>\n<\/div>\n<p>The same molar volume can be found with a different gas and a different collecting vessel, and this version is the one most often set on the OCR A course.<\/p>\n<p>A weighed length of <strong>magnesium ribbon<\/strong> reacts with an excess of hydrochloric acid and the hydrogen is collected in a <strong>100 cm\u00b3 gas syringe<\/strong>.<\/p>\n<p class=\"ols-equation\">Mg(s) + 2HCl(aq) \u2192 MgCl\u2082(aq) + H\u2082(g)<\/p>\n<p>The ratio is again 1 : 1, so the amount of hydrogen equals the amount of magnesium (Ar = 24.3). Magnesium is far lighter per mole than calcium carbonate, so the masses are small.<\/p>\n<p>0.10 g of magnesium is 0.0041 mol and would give about 99 cm\u00b3 of hydrogen, so the runs use about 0.02 g to 0.09 g and the ribbon is cleaned with emery paper first to remove the oxide layer.<\/p>\n<p>Weighing by difference uses a weighing boat rather than a test tube.<\/p>\n<p>The acid check is the same shape: 25 cm\u00b3 of 1.0 mol dm\u207b\u00b3 HCl is 0.025 mol, and 0.09 g of magnesium (0.0037 mol) needs 0.0074 mol, so the acid is in excess.<\/p>\n<p>The syringe removes the dissolving error and the water-vapour error at once, so the result is usually closer to the accepted value. It brings its own points for the evaluation.<\/p><p>The plunger must move freely: a <strong>sticking plunger<\/strong> gives a volume that is too low and a syringe left tilted can let the plunger creep out.<\/p><p>The syringe scale is in 1 cm\u00b3 divisions, so each reading carries \u00b10.5 cm\u00b3 (some mark schemes accept \u00b11 cm\u00b3 because the plunger seal is wide). The gas must still be allowed to cool to room temperature before the reading.<\/p><p>Because the reaction is faster than carbonate and acid, the delay in fitting the bung matters even more. A better arrangement suspends the magnesium in a small tube inside the flask and tips it in after the bung is fitted.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Feature<\/th><th>Carbon dioxide over water<\/th><th>Hydrogen in a gas syringe<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Reaction<\/strong><\/td><td>CaCO\u2083(s) + 2CH\u2083COOH(aq)<\/td><td>Mg(s) + 2HCl(aq)<\/td><\/tr>\n<tr><td><strong>Mass range<\/strong><\/td><td>0.05 g to 0.40 g of CaCO\u2083 (Mr 100.1)<\/td><td>0.02 g to 0.09 g of Mg (Ar 24.3)<\/td><\/tr>\n<tr><td><strong>Largest error<\/strong><\/td><td>Gas escaping before the bung is fitted; CO\u2082 dissolving in water<\/td><td>Gas escaping before the bung is fitted; sticking plunger<\/td><\/tr>\n<tr><td><strong>Reading<\/strong><\/td><td>\u00b10.5 cm\u00b3 at the water level, cylinder vertical<\/td><td>\u00b10.5 cm\u00b3 at the plunger seal, syringe horizontal<\/td><\/tr>\n<tr><td><strong>Expected result<\/strong><\/td><td>Slightly below 24.0 dm\u00b3 mol\u207b\u00b9<\/td><td>Close to 24.0 dm\u00b3 mol\u207b\u00b9 at room temperature and pressure, taken as 293 K and 101 kPa<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;A gas syringe was used because hydrogen is collected directly, so none is lost by dissolving and no water vapour is added to the volume.&#8221;<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">11<\/div>\n<h2>Common Mistakes<\/h2>\n<\/div>\n<p>The same errors appear in students\u2019 write-ups and exam answers year after year. Each one costs a mark that the corrected version earns.<\/p>\n<ul>\n<li>Writing &#8220;the volume of one mole of gas is 24 dm\u00b3&#8221; with <strong>no conditions<\/strong>. The value is only meaningful at a stated temperature and pressure.<\/li>\n<li>Quoting the molar volume as 24.0 dm\u00b3 mol\u207b\u00b9 from a volume read to 2 significant figures. Match the <strong>significant figures<\/strong> to the least precise measurement.<\/li>\n<li>Using the mass of powder weighed out instead of the <strong>mass by difference<\/strong>. Some powder always stays in the test tube.<\/li>\n<li>Reading a single run instead of the line, or drawing the line through the anomalous point instead of ignoring it.<\/li>\n<li>Saying the first bubbles should be discarded &#8220;because they are air&#8221;. They are, and they are matched by gas left in the apparatus at the end.<\/li>\n<li>Giving &#8220;human error&#8221; or &#8220;the equipment was inaccurate&#8221; as an error. Name the source, its <strong>direction<\/strong> and a specific improvement.<\/li>\n<li>Calculating percentage uncertainty with a made-up uncertainty. Use the instrument\u2019s actual value: \u00b10.005 g per balance reading, \u00b10.5 cm\u00b3 for the cylinder.<\/li>\n<li>Forgetting the <strong>2 : 1<\/strong> ratio in the excess check and comparing 0.030 mol of acid with 0.0040 mol of carbonate directly.<\/li>\n<\/ul>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Graphs, Gas Syringes and Conditions<\/h2>\n<p>Pick the accurate statement in each round; one word usually makes a statement wrong.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-983\" class=\"h5p-iframe\" data-content-id=\"983\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Molar Volume of a Gas Summary: Graphs, Gas Syringes and Conditions\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">12<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3><p>&#8220;Weighing the test tube before and after tipping gives the mass of CaCO\u2083 that actually reacted.&#8221;<\/p><p>&#8220;The line passes through the origin because no solid gives no gas, and reading from it at 0.25 g averages out random error.&#8221;<\/p><p>&#8220;The molar volume is 24 dm\u00b3 mol\u207b\u00b9 at room temperature and pressure (2 s.f.); CO\u2082 escaping before the bung was fitted and dissolving in the water make it too low.&#8221;<\/p><p>&#8220;A gas syringe avoids the loss of CO\u2082 by dissolving.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;The acid must be exactly the right amount&#8221; (it must be in excess so that CaCO\u2083 is the limiting reagent).<\/p><p>&#8220;Discard the first bubbles because they are air&#8221; (keep them; an equal volume of CO\u2082 stays in the apparatus).<\/p><p>&#8220;The result was wrong because of human error&#8221; (name the source and its direction). &#8220;24 dm\u00b3&#8221; with no conditions.<\/p>\n<h3>Watch for<\/h3><p>The excess check needs the 2 : 1 ratio. Percentage error uses the accepted value as the denominator. Percentage uncertainty of a mass by difference uses two readings, \u00b10.01 g.<\/p>\n<p>The independent variable (mass) goes on the x-axis. Water vapour is the one error that makes the volume too high; every loss of gas makes it too low.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Quick answers to the questions students ask most about measuring the molar volume of a gas: fitting the bung, the graph, the first bubbles and the conditions.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why must the bung be fitted so quickly?<\/h3>\n<p>The reaction is fastest in the first few seconds, so gas produced before the bung is in escapes to the room.<\/p>\n<p>The volume collected is then too low and so is the calculated molar volume, and because the loss cannot be measured it cannot be corrected for.<\/p>\n<p>A better arrangement suspends the solid inside the sealed tube and tips it in afterwards.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the line of best fit drawn through the origin?<\/h3>\n<p>If no calcium carbonate reacts, no carbon dioxide is produced, so the point (0 g, 0 cm\u00b3) is a known point on the graph. The line is fixed at the origin and then angled so that the measured points are scattered evenly either side of it, ignoring any anomaly.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why read the graph at 0.25 g instead of using one of the runs?<\/h3>\n<p>0.25 g lies inside the range of masses measured, so the reading comes from the line of best fit rather than from a single run.<\/p><p>The line averages the random errors of all seven runs and ignores the anomalous one, so the volume read at 0.25 g (about 60 cm\u00b3) is more reliable than any single measurement.<\/p><p>Any mass on the line would give the same molar volume, because the line has one gradient.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why are the first bubbles not discarded?<\/h3>\n<p>They are air pushed out of the delivery tube and the space above the acid by the gas being made.<\/p><p>At the end of the run that same space is full of carbon dioxide that never reached the cylinder, and its volume equals the volume of air pushed out at the start.<\/p><p>Keeping the first bubbles lets the two cancel; discarding them makes the volume too low.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why record room temperature and pressure?<\/h3>\n<p>The volume of a fixed amount of gas depends on both. One mole occupies about 24.0 dm\u00b3 at 20 \u00b0C and 101 kPa but 24.8 dm\u00b3 at 25 \u00b0C and 100 kPa, so a molar volume without its conditions cannot be compared with anything.<\/p><p>The ideal gas equation pV = nRT gives the expected value at the recorded conditions.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>When is a gas syringe better than collecting over water?<\/h3>\n<p>Whenever the gas dissolves in water: carbon dioxide slightly, ammonia, hydrogen chloride and sulfur dioxide almost completely.<\/p><p>The syringe collects the gas directly, so nothing is lost by dissolving and no water vapour is added to the volume. Its own weaknesses are a sticking plunger and a scale read only to the nearest 1 cm\u00b3.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Practical Activity Groups Pages<\/h2>\n<p>Use these pages to connect the practical techniques with each other and with the rest of the course.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-3-enthalpy-determination\/\">PAG 3: Enthalpy Change via Hess&#8217;s Law<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-acid-base-titration\/\">PAG 2: Concentration of HCl by Titration<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-2-preparing-a-standard-solution\/\">PAG 2: Preparation of a Standard Solution<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/rates-of-hydrolysis-of-haloalkanes\/\">Hydrolysis rates: Hydrolysis of Halogenoalkanes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-5-synthesis-of-an-organic-liquid\/\">PAG 5: Chlorination of 2-methylpropan-2-ol<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-5-oxidation-of-an-alcohol\/\">PAG 5: Oxidation of an Alcohol<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/pag-4-pag-7-qualitative-analysis-of-ions-and-functional-groups\/\">PAG 4 and 7: Analysis of Inorganic and Organic Unknowns<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/practical-activity-groups\/\">Practical Activity Groups (PAGs) Overview<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright and author footprint:<\/strong> This OLS revision page was written for Online Learning System by <strong>Dr. Mohammed Al-Fatah<\/strong>. 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