{"id":8058,"date":"2026-09-15T08:23:46","date_gmt":"2026-09-15T07:23:46","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/calculations-involving-moles\/"},"modified":"2026-09-19T20:33:04","modified_gmt":"2026-09-19T19:33:04","slug":"calculations-involving-moles","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/calculations-involving-moles\/","title":{"rendered":"Calculations involving Moles"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. All rights reserved.\n\nThis WordPress revision page HTML, CSS, content sequence, educational wording,\nlayout structure, schema structure and embedded design logic are protected\nintellectual property of Online Learning System.\n\nUnauthorised copying, redistribution, resale, modification, republication,\nscraping, extraction, derivative reuse, automated harvesting or removal of\ncopyright notices is strictly prohibited.\n\nBackend copyright marker:\nOLS-T1-CALCULATIONS-INVOLVING-MOLES-REVISION-PAGE-2026\n\nPage:\nCalculations Involving Moles\nOCR A Level Chemistry A\nPaper 1 and Paper 2\nAmount of Substance\nH432\/01 and H432\/02\n\nCopyright enforcement notes:\n- The visible student page is a free OLS revision resource.\n- The backend HTML structure, CSS architecture, card sequencing, schema graph,\n  revision wording, responsive table behaviour and embedded learning pathway are\n  proprietary OLS production assets.\n- Do not remove this notice.\n===============================================================================\n-->\n\n<section class=\"ols-revision-page ols-calculations-involving-moles-page\" data-owner=\"Online Learning System\" data-copyright=\"\u00a9 Online Learning System. 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/atoms-elements-and-molecules\/\">Atoms, Elements and Molecules<\/a><\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/\">Mole Calculations<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/comparing-masses-of-substances\/\">Comparing Masses of Substances<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/calculations-involving-moles\/\">Calculations involving Moles<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/calculations-using-reacting-masses\/\">Calculations using Reacting Masses<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/the-yield-of-a-reaction\/\">The Yield of a Reaction<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/atom-economy\/\">Atom Economy<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/\">Empirical and Molecular Formulae<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/empirical-formulae\/\">Empirical Formulae<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/molecular-formulae\/\">Molecular Formulae<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/\">Calculations with Solutions and Gases<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/molar-volume-calculations\/\">Molar Volume Calculations<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/concentrations-of-solutions\/\">Concentrations of Solutions<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/hydrated-salts-and-water-of-crystallisation\/\">Hydrated Salts and Water of Crystallisation<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/measuring-mass-volume-and-gas-volume\/\">Measuring Mass, Volume and Gas Volume<\/a><\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Other OCR Sections<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-1-atomic-structure-and-isotopes\/\">2.1.1 Atomic Structure and Isotopes<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-2-compounds-formulae-and-equations\/\">2.1.2 Compounds, Formulae and Equations<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/\">2.1.4 Acids<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-1-electron-structure\/\">2.2.1 Electron Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/\">2.2.2 Bonding and Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-1-1-periodicity\/\">3.1.1 Periodicity<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-1-basic-concepts-of-organic-chemistry\/\">4.1.1 Basic Concepts of Organic Chemistry<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-2-alkanes\/\">4.1.2 Alkanes<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-3-alkenes\/\">4.1.3 Alkenes<\/a><\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = 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\/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/\">2.1.3 Amount of Substance<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/\">Mole Calculations<\/a> \/\n<span>Calculations involving Moles<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Calculations Involving Moles<\/h1>\n        <p class=\"ols-page-intro\">A focused revision guide to moles, molar mass, mole-mass calculations and the Avogadro constant. These ideas are used throughout quantitative chemistry, so the aim is to make the meaning of one mole clear before applying it to calculations.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1 and 2<\/div>\n<div class=\"ols-badge\">2.1.3 Amount of Substance<\/div>\n<div class=\"ols-badge\">H432\/01 and H432\/02<\/div>\n<\/div>\n\n        <div class=\"ols-author\">\n          <img decoding=\"async\" class=\"ols-author-avatar-img\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\" alt=\"Dr. Mohammed Al-Fatah\">\n          <div class=\"ols-author-content\">\n            <h2 class=\"ols-author-title\">Written by:<br><span>Dr. Mohammed Al-Fatah<\/span><\/h2>\n            <p class=\"ols-author-description\">Chemistry specialist revision notes for A Level Chemistry.<\/p>\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\"><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\"><\/path><\/svg>\n              View LinkedIn Profile\n            <\/a>\n          <\/div>\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: Relative Formula Mass<\/h2>\n<p>Before you start, check that you can still work out a relative formula mass the GCSE way.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"614\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">1<\/div><h2>The Mole<\/h2><\/div>\n        <p>In chemistry, a <strong>mole<\/strong> is a measure of <strong>amount of substance<\/strong>. It lets chemists count particles by weighing substances, rather than trying to count individual atoms, ions or molecules directly.<\/p>\n        <p>The important point is that the formula must always be stated. For example, <strong>1 mole of oxygen atoms, O<\/strong>, is not the same as <strong>1 mole of oxygen molecules, O<sub>2<\/sub><\/strong>.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Mole:<\/strong> the SI unit for amount of substance. The abbreviation for mole is <strong>mol<\/strong>.<\/p><\/div>\n        <div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> always quote the formula when referring to a mole of a substance, because different formulae have different molar masses.<\/p><\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">2<\/div><h2>Why the Formula Must Be Quoted<\/h2><\/div>\n        <p>If a formula is not stated, the meaning of the calculation can become ambiguous. This is especially important for elements that exist as molecules and for hydrated salts.<\/p>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Expression<\/th>\n                <th>Meaning<\/th>\n                <th>Molar mass used<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Expression\">1 mol of oxygen atoms, O<\/td>\n                <td data-label=\"Meaning\">One mole of separate oxygen atoms.<\/td>\n                <td data-label=\"Molar mass used\">16 g mol<sup>-1<\/sup><\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Expression\">1 mol of oxygen molecules, O<sub>2<\/sub><\/td>\n                <td data-label=\"Meaning\">One mole of oxygen molecules, each containing two oxygen atoms.<\/td>\n                <td data-label=\"Molar mass used\">32 g mol<sup>-1<\/sup><\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Expression\">1 mol of anhydrous copper(II) sulfate, CuSO<sub>4<\/sub><\/td>\n                <td data-label=\"Meaning\">Copper(II) sulfate with no water of crystallisation.<\/td>\n                <td data-label=\"Molar mass used\">160 g mol<sup>-1<\/sup><\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Expression\">1 mol of hydrated copper(II) sulfate, CuSO<sub>4<\/sub>.5H<sub>2<\/sub>O<\/td>\n                <td data-label=\"Meaning\">Copper(II) sulfate crystals containing water of crystallisation.<\/td>\n                <td data-label=\"Molar mass used\">250 g mol<sup>-1<\/sup><\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">3<\/div><h2>Working Out Masses<\/h2><\/div>\n        <p>The mass of one mole of a substance is found by working out its <strong>relative formula mass<\/strong>, then attaching the unit <strong>grams<\/strong>.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Rule:<\/strong> mass of 1 mole of a substance = relative formula mass in grams.<\/p><\/div>\n        <p>For example, iron(II) sulfate crystals have the formula <strong>FeSO<sub>4<\/sub>.7H<sub>2<\/sub>O<\/strong>. Using H = 1, O = 16, S = 32 and Fe = 56:<\/p>\n        <div class=\"ols-key-box\"><p><strong>RFM of FeSO<sub>4<\/sub>.7H<sub>2<\/sub>O:<\/strong> 56 + 32 + (4 \u00d7 16) + 7 \u00d7 [(2 \u00d7 1) + 16] = 278. Therefore, 1 mol of FeSO<sub>4<\/sub>.7H<sub>2<\/sub>O has a mass of 278 g.<\/p><\/div>\n        <p>For oxygen gas, the formula is <strong>O<sub>2<\/sub><\/strong>, so the relative formula mass is <strong>2 \u00d7 16 = 32<\/strong>. Therefore, 1 mol of O<sub>2<\/sub> has a mass of 32 g.<\/p>\n      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Which Formula, Which Mass?<\/h2>\n<p>Pick the mass of one mole for each formula as quickly as you can.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-615\" class=\"h5p-iframe\" data-content-id=\"615\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Mole Calculations Quick-Fire: Molar Mass Depends on the Formula\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">4<\/div><h2>Simple Calculations with Moles<\/h2><\/div>\n        <p>The most common mole calculation links <strong>mass<\/strong>, <strong>amount of substance<\/strong> and <strong>molar mass<\/strong>.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Core equation:<\/strong> amount of substance = mass \u00f7 molar mass<\/p><\/div>\n        <p>In symbols, this is usually written as:<\/p>\n        <div class=\"ols-key-box\"><p><strong>n = m \u00f7 M<\/strong>, where <strong>n<\/strong> is amount in mol, <strong>m<\/strong> is mass in g and <strong>M<\/strong> is molar mass in g mol<sup>-1<\/sup>.<\/p><\/div>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Question type<\/th>\n                <th>Method<\/th>\n                <th>Example<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Question type\">Find mass from moles<\/td>\n                <td data-label=\"Method\">mass = amount \u00d7 molar mass<\/td>\n                <td data-label=\"Example\">0.200 mol of CaCO<sub>3<\/sub> has mass 0.200 \u00d7 100 = 20.0 g<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Question type\">Find moles from mass<\/td>\n                <td data-label=\"Method\">amount = mass \u00f7 molar mass<\/td>\n                <td data-label=\"Example\">54 g of H<sub>2<\/sub>O is 54 \u00f7 18 = 3.00 mol<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Question type\">Find molar mass<\/td>\n                <td data-label=\"Method\">molar mass = mass \u00f7 amount<\/td>\n                <td data-label=\"Example\">10.0 g of a substance containing 0.250 mol has M = 40.0 g mol<sup>-1<\/sup><\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\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>Quick Check: Mass, Amount and Molar Mass<\/h2>\n<p>Work each calculation out on paper before you turn the card.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-293\" class=\"h5p-iframe\" data-content-id=\"293\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Moles and Mass Calculations Practice\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <div class=\"ols-zoom-card\">\n        <div class=\"ols-zoom-card-image\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Chemistry-mole-mass-formula-guide.webp\" alt=\"Chemistry mole mass formula guide showing the relationship between mass, amount and molar mass\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-caption\">\n          <p>The mole-mass triangle helps students rearrange the relationship between mass, amount and molar mass.<\/p>\n        <\/div>\n      <\/div>\n\n      \n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>Avogadro Constant<\/h2><\/div>\n        <p>The <strong>Avogadro constant<\/strong> is the number of specified particles in 1 mole of a substance. At A Level, this is often taken as approximately <strong>6.02 \u00d7 10<sup>23<\/sup> mol<sup>-1<\/sup><\/strong>.<\/p>\n        <p>The particles being counted depend on the formula being discussed. You may be counting atoms, molecules, ions or formula units.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Avogadro constant:<\/strong> the number of particles per mole, approximately 6.02 \u00d7 10<sup>23<\/sup> mol<sup>-1<\/sup>.<\/p><\/div>\n        <div class=\"ols-key-box\"><p><strong>Core equation:<\/strong> number of particles = amount of substance \u00d7 Avogadro constant.<\/p><\/div>\n      <\/article>\n\n      <div class=\"ols-zoom-card\">\n        <div class=\"ols-zoom-card-image\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/The-Avogadro-constant-explained.webp\" alt=\"The Avogadro constant explained using one mole and number of particles\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-caption\">\n          <p>The Avogadro constant connects a measurable amount in moles to the number of particles present.<\/p>\n        <\/div>\n      <\/div>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Using Avogadro Constant in Calculations<\/h2><\/div>\n        <p>To calculate the number of particles, first work out the amount in moles. Then multiply by the Avogadro constant.<\/p>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Example<\/th>\n                <th>Working<\/th>\n                <th>Answer<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Example\">How many atoms are in 6.00 g of tin, Sn?<\/td>\n                <td data-label=\"Working\">amount = 6.00 \u00f7 118.7 = 0.0505 mol. Number of atoms = 0.0505 \u00d7 6.02 \u00d7 10<sup>23<\/sup><\/td>\n                <td data-label=\"Answer\">3.04 \u00d7 10<sup>22<\/sup> atoms<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Example\">How many molecules are in 9.00 g of water, H<sub>2<\/sub>O?<\/td>\n                <td data-label=\"Working\">amount = 9.00 \u00f7 18.0 = 0.500 mol. Number of molecules = 0.500 \u00d7 6.02 \u00d7 10<sup>23<\/sup><\/td>\n                <td data-label=\"Answer\">3.01 \u00d7 10<sup>23<\/sup> molecules<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Example\">How many chloride ions are in 0.0100 mol of MgCl<sub>2<\/sub>?<\/td>\n                <td data-label=\"Working\">Each formula unit of MgCl<sub>2<\/sub> contains two chloride ions, so chloride ions = 0.0100 \u00d7 2 \u00d7 6.02 \u00d7 10<sup>23<\/sup><\/td>\n                <td data-label=\"Answer\">1.20 \u00d7 10<sup>22<\/sup> chloride ions<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> read the question carefully. A question may ask for molecules, total atoms, ions or formula units, and these are not always the same thing.<\/p><\/div>\n      <\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: The Route from Mass to Atoms<\/h2>\n<p>Drag the steps of the calculation into the right order.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-294\" class=\"h5p-iframe\" data-content-id=\"294\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Avogadro Constant Put in Order: From Mass to Number of Atoms\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Masses and Particles<\/h2>\n<p>Type each answer to 3 significant figures.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-295\" class=\"h5p-iframe\" data-content-id=\"295\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Avogadro Constant Fill in the Blanks: Masses and Particles\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">7<\/div><h2>Defining the Mole<\/h2><\/div>\n        <p>A mole of substance contains the same number of specified elementary units as there are atoms in 12 g of carbon-12.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Definition:<\/strong> a mole of substance is the amount of that substance that contains the same number of stated elementary units as there are atoms in 12 g of <sup>12<\/sup>C.<\/p><\/div>\n        <p>The phrase <strong>stated elementary units<\/strong> simply means the particles you are choosing to count. These can be atoms, molecules, ions or formula units.<\/p>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Substance described<\/th>\n                <th>Stated elementary units<\/th>\n                <th>What 1 mol contains<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Substance described\">Oxygen atoms, O<\/td>\n                <td data-label=\"Stated elementary units\">Atoms<\/td>\n                <td data-label=\"What 1 mol contains\">6.02 \u00d7 10<sup>23<\/sup> oxygen atoms<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Substance described\">Oxygen molecules, O<sub>2<\/sub><\/td>\n                <td data-label=\"Stated elementary units\">Molecules<\/td>\n                <td data-label=\"What 1 mol contains\">6.02 \u00d7 10<sup>23<\/sup> oxygen molecules<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Substance described\">Sodium chloride, NaCl<\/td>\n                <td data-label=\"Stated elementary units\">Formula units<\/td>\n                <td data-label=\"What 1 mol contains\">6.02 \u00d7 10<sup>23<\/sup> NaCl formula units<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Substance described\">Magnesium ions, Mg<sup>2+<\/sup><\/td>\n                <td data-label=\"Stated elementary units\">Ions<\/td>\n                <td data-label=\"What 1 mol contains\">6.02 \u00d7 10<sup>23<\/sup> Mg<sup>2+<\/sup> ions<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\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>Quick Check: Counting Ions<\/h2>\n<p>Decide exactly which particles the question is asking you to count.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-330\" class=\"h5p-iframe\" data-content-id=\"330\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Total number of ions in 10.0 g of iron III sulfate\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Which Statement Is Right?<\/h2>\n<p>In each round, choose the one correct statement about moles and particles.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-616\" class=\"h5p-iframe\" data-content-id=\"616\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"The Mole Summary: Counting the Stated Particles\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <div class=\"ols-zoom-card\">\n        <div class=\"ols-zoom-card-image\">\n          <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Breaking-down-Avogadros-number-explained.webp\" alt=\"Breaking down Avogadro's number using particles in one mole\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-caption\">\n          <p>One mole always contains the Avogadro number of the specified particles, but the particle type must be identified.<\/p>\n        <\/div>\n      <\/div>\n\n      \n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">8<\/div><h2>Common Exam Points<\/h2><\/div>\n        <ul>\n          <li>A mole is a measure of amount of substance.<\/li>\n          <li>The abbreviation for mole is mol.<\/li>\n          <li>Always quote the formula when stating the amount of a substance.<\/li>\n          <li>The mass of 1 mole of a substance is its relative formula mass in grams.<\/li>\n          <li>Use <strong>n = m \u00f7 M<\/strong> for mole-mass calculations.<\/li>\n          <li>Use <strong>number of particles = amount \u00d7 Avogadro constant<\/strong> for particle calculations.<\/li>\n          <li>The Avogadro constant is approximately 6.02 \u00d7 10<sup>23<\/sup> mol<sup>-1<\/sup>.<\/li>\n          <li>Check whether the question asks for atoms, molecules, ions, formula units or total atoms.<\/li>\n          <li>For ionic compounds such as NaCl, use the term formula units rather than molecules.<\/li>\n          <li>For hydrated salts, include water of crystallisation in the formula mass.<\/li>\n        <\/ul>\n      <\/article>\n\n      \n\n      <section class=\"ols-quicksnap-card\">\n        <h2>QuickSnap<\/h2>\n        <p>This text summary condenses the page into the essential exam ideas.<\/p>\n        <ul class=\"ols-quicksnap-list\">\n          <li><strong>Mole:<\/strong> a measure of amount of substance.<\/li>\n          <li><strong>Formula matters:<\/strong> O and O<sub>2<\/sub> have different molar masses.<\/li>\n          <li><strong>Mass of 1 mol:<\/strong> relative formula mass written in grams.<\/li>\n          <li><strong>Mole-mass equation:<\/strong> n = m \u00f7 M.<\/li>\n          <li><strong>Avogadro constant:<\/strong> approximately 6.02 \u00d7 10<sup>23<\/sup> mol<sup>-1<\/sup>.<\/li>\n          <li><strong>Particle equation:<\/strong> number of particles = amount \u00d7 Avogadro constant.<\/li>\n          <li><strong>Particle type:<\/strong> identify whether the question means atoms, molecules, ions or formula units.<\/li>\n        <\/ul>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxCalculationsInvolvingMoles001\" aria-hidden=\"true\" role=\"dialog\" aria-modal=\"true\" aria-label=\"Expanded revision image\">\n        <div class=\"ols-image-lightbox-inner\">\n          <button class=\"ols-image-lightbox-close\" type=\"button\" aria-label=\"Close enlarged image\">\u00d7<\/button>\n          <img 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It represents a fixed number of specified particles, such as atoms, molecules, ions or formula units.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why must I quote the formula when using moles?<\/h3>\n            <p>The formula tells you exactly what particles or substance are being counted. For example, 1 mol of oxygen atoms, O, has a mass of 16 g, while 1 mol of oxygen molecules, O<sub>2<\/sub>, has a mass of 32 g.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>How do I calculate the number of moles from mass?<\/h3>\n            <p>Use amount = mass \u00f7 molar mass. The mass should be in grams and the molar mass should be in g mol<sup>-1<\/sup>.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What is the Avogadro constant?<\/h3>\n            <p>The Avogadro constant is the number of particles in 1 mole of a substance. It is approximately 6.02 \u00d7 10<sup>23<\/sup> mol<sup>-1<\/sup>.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What is the difference between molecules and formula units?<\/h3>\n            <p>Molecules are discrete covalent particles, such as H<sub>2<\/sub>O or O<sub>2<\/sub>. Formula units describe the simplest ratio of ions in an ionic compound, such as NaCl or MgCl<sub>2<\/sub>.<\/p>\n          <\/div>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Build the surrounding calculation skills needed for quantitative chemistry.<\/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\/2-1-3-amount-of-substance\/mole-calculations\/comparing-masses-of-substances\/\">Comparing Masses of Substances<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/mole-calculations\/calculations-using-reacting-masses\/\">Calculations Using Reacting Masses<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/molar-volume-calculations\/\">Molar Volume Calculations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/calculations-with-solutions-and-gases\/concentrations-of-solutions\/\">Concentrations of Solutions<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/empirical-and-molecular-formulae\/empirical-formulae\/\">Empirical Formulae<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-3-amount-of-substance\/\">this section Overview<\/a>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-attribution-card\">\n        <p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. 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