{"id":8078,"date":"2026-09-15T08:24:23","date_gmt":"2026-09-15T07:24:23","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/comparing-masses-of-substances\/"},"modified":"2026-09-19T20:33:52","modified_gmt":"2026-09-19T19:33:52","slug":"comparing-masses-of-substances","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/comparing-masses-of-substances\/","title":{"rendered":"Comparing Masses of Substances"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/\">Topic overview<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/atoms-elements-and-molecules\/\">2.1 \/ 2.2 Atoms, Elements and Molecules<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/ions-and-ionic-formulae\/\">2.3 Ions and Ionic Formulae<\/a><\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/\">2.3 Equations and Reaction Types<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/writing-chemical-equations\/\">Writing Chemical Equations<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/typical-reactions-of-acids\/\">Typical Reactions of Acids<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/equations-and-reaction-types\/ionic-and-full-equations\/\">Ionic and Full Equations<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/\">2.3 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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/empirical-formulae\/\">Empirical Formulae<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/molecular-formulae\/\">Molecular Formulae<\/a><\/li>\n<li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/hydrated-salts-and-water-of-crystallisation\/\">2.3 Hydrated Salts and Water of Crystallisation<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/\">2.4 Mole Calculations<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/calculations-involving-moles\/\">Calculations involving Moles<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/\">2.4 Solutions and Gases<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/calculations-with-solutions-and-gases\/titration-calculations\/\">Titration Calculations<\/a><\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Other Topics<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-1-atomic-structure\/\">Topic 1 Atomic Structure<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-3-chemical-bonding\/\">Topic 3 Chemical Bonding<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-4-states-of-matter\/\">Topic 4 States of Matter<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-9-the-periodic-table-chemical-periodicity\/\">Topic 9 Chemical Periodicity<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-13-an-introduction-to-as-level-organic-chemistry\/\">Topic 13 Introduction to Organic Chemistry<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-14-hydrocarbons\/\">Topic 14 Hydrocarbons<\/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  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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\/cie\/\">Cambridge International (CIE)<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/\">Topic 2 Atoms, Molecules and Stoichiometry<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/\">Mole Calculations<\/a> \/\n<span>Comparing Masses of Substances<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Comparing Masses of Substances<\/h1>\n        <p class=\"ols-page-intro\">A focused revision guide to relative atomic mass, relative formula mass, percentage composition and hydrated salts. These ideas allow chemists to compare masses of atoms, compounds and water of crystallisation using the carbon-12 scale.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">AS Level<\/div>\n<div class=\"ols-badge\">Topic 2: Atoms, Molecules and Stoichiometry<\/div>\n<div class=\"ols-badge\">9701 Papers 1 and 2<\/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: Counting Atoms in a Formula<\/h2>\n<p>Before you start, check that you can count the atoms in a formula that contains brackets.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"622\"><\/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>Relative Atomic Mass<\/h2><\/div>\n        <p>Atoms are far too small to measure conveniently in grams during ordinary chemical calculations. Instead, atomic masses are compared with the mass of an atom of the <strong>carbon-12 isotope<\/strong>.<\/p>\n        <p>The carbon-12 scale defines one atomic mass unit as <strong>1\/12 of the mass of one carbon-12 atom<\/strong>. On this scale, a carbon-12 atom has a mass of exactly 12 units.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Relative atomic mass, A<sub>r<\/sub>:<\/strong> the weighted mean mass of the atoms of an element compared with 1\/12 of the mass of a carbon-12 atom.<\/p><\/div>\n        <p>The word <strong>weighted<\/strong> matters. Most elements exist as a mixture of isotopes, so the relative atomic mass takes account of both the mass of each isotope and its relative abundance.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> relative atomic mass has no unit because it is a comparison with the carbon-12 standard.<\/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: Reading a Weighted Mean<\/h2>\n<p>Use the idea of a weighted mean to decide which statement about copper must be true.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-623\" class=\"h5p-iframe\" data-content-id=\"623\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Relative Atomic Mass MCQ: What a Weighted Mean Tells You\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">2<\/div><h2>Relative Formula Mass<\/h2><\/div>\n        <p>The <strong>relative formula mass<\/strong>, M<sub>r<\/sub>, is found by adding together the relative atomic masses of all the atoms shown in a formula.<\/p>\n        <p>The term <strong>relative molecular mass<\/strong>, sometimes written RMM, should only be used for substances that exist as molecules. <strong>Relative formula mass<\/strong> is safer because it applies to molecular substances, ionic compounds and giant structures.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Relative formula mass, M<sub>r<\/sub>:<\/strong> the sum of the relative atomic masses of all atoms in the formula of a substance.<\/p><\/div>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Substance<\/th>\n                <th>Formula<\/th>\n                <th>Calculation<\/th>\n                <th>M<sub>r<\/sub><\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Substance\">Carbon dioxide<\/td>\n                <td data-label=\"Formula\">CO<sub>2<\/sub><\/td>\n                <td data-label=\"Calculation\">12.0 + (2 \u00d7 16.0)<\/td>\n                <td data-label=\"Mr\">44.0<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Substance\">Calcium carbonate<\/td>\n                <td data-label=\"Formula\">CaCO<sub>3<\/sub><\/td>\n                <td data-label=\"Calculation\">40.1 + 12.0 + (3 \u00d7 16.0)<\/td>\n                <td data-label=\"Mr\">100.1<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Substance\">Magnesium nitrate<\/td>\n                <td data-label=\"Formula\">Mg(NO<sub>3<\/sub>)<sub>2<\/sub><\/td>\n                <td data-label=\"Calculation\">24.3 + 2(14.0 + 3 \u00d7 16.0)<\/td>\n                <td data-label=\"Mr\">148.3<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n        <div class=\"ols-key-box\"><p><strong>Exam focus:<\/strong> brackets multiply everything inside them. In Mg(NO<sub>3<\/sub>)<sub>2<\/sub>, there are two nitrogen atoms and six oxygen atoms.<\/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: Spot the Wrong Mr Values<\/h2>\n<p>Check each relative formula mass and click every value that is wrong.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-624\" class=\"h5p-iframe\" data-content-id=\"624\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Relative Formula Mass Mark the Words: Find the Wrong Values\"><\/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\/How-to-calculate-relative-formula-masses.webp\" alt=\"How to calculate relative formula masses using relative atomic masses\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-caption\">\n          <p>Relative formula mass is calculated by counting each atom in the formula and adding the correct relative atomic masses.<\/p>\n        <\/div>\n      <\/div>\n\n      \n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">3<\/div><h2>Using M<sub>r<\/sub> to Find Percentage Composition<\/h2><\/div>\n        <p><strong>Percentage composition<\/strong> tells you what percentage of the mass of a compound comes from a particular element.<\/p>\n        <p>First calculate the M<sub>r<\/sub> of the compound. Then work out the total mass contribution from the element being considered.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Percentage by mass of an element:<\/strong> (total A<sub>r<\/sub> of that element in the formula \u00f7 M<sub>r<\/sub> of the compound) \u00d7 100<\/p><\/div>\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>Example: urea, CO(NH<sub>2<\/sub>)<sub>2<\/sub><\/h3>\n            <p>M<sub>r<\/sub> = 12.0 + 16.0 + (2 \u00d7 14.0) + (4 \u00d7 1.0) = 60.0<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Percentage of nitrogen in urea<\/h3>\n            <p>Nitrogen contributes 28.0 out of 60.0, so percentage nitrogen = (28.0 \u00f7 60.0) \u00d7 100 = <strong>46.7%<\/strong>.<\/p>\n          <\/div>\n        <\/div>\n        <div class=\"ols-key-box\"><p><strong>Common mistake:<\/strong> do not use the A<sub>r<\/sub> of a single atom if the formula contains more than one atom of that element.<\/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: Percentage by Mass<\/h2>\n<p>Answer three questions on percentage composition, including one that runs the formula backwards.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"329\"><\/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\/Calculating-percentage-composition-in-chemistry.webp\" alt=\"Calculating percentage composition in chemistry from relative formula mass\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-caption\">\n          <p>Percentage composition compares the mass contribution of one element with the total relative formula mass of the compound.<\/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\">4<\/div><h2>Hydrated Salts<\/h2><\/div>\n        <p>A <strong>hydrated salt<\/strong> is an ionic compound that contains water of crystallisation in its solid crystal structure. This water is shown using a dot in the formula, such as CuSO<sub>4<\/sub>\u00b75H<sub>2<\/sub>O.<\/p>\n        <p>When a hydrated salt is heated strongly, the water of crystallisation can be driven off. The remaining solid is called the <strong>anhydrous salt<\/strong>.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Hydrated salt:<\/strong> a crystalline salt that contains a fixed number of water molecules chemically incorporated into its crystal structure.<\/p><\/div>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Term<\/th>\n                <th>Meaning<\/th>\n                <th>Example<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Term\">Hydrated<\/td>\n                <td data-label=\"Meaning\">Contains water of crystallisation.<\/td>\n                <td data-label=\"Example\">CuSO<sub>4<\/sub>\u00b75H<sub>2<\/sub>O<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Term\">Anhydrous<\/td>\n                <td data-label=\"Meaning\">Contains no water of crystallisation.<\/td>\n                <td data-label=\"Example\">CuSO<sub>4<\/sub><\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Term\">Dot formula<\/td>\n                <td data-label=\"Meaning\">Shows the ratio of salt formula units to water molecules.<\/td>\n                <td data-label=\"Example\">ZnSO<sub>4<\/sub>\u00b77H<sub>2<\/sub>O<\/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: Reading a Dot Formula<\/h2>\n<p>Type the answer to each question about a hydrated salt.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-291\" class=\"h5p-iframe\" data-content-id=\"291\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrated Salts Flashcards: Reading a Dot Formula\"><\/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\/Heating-in-a-crucible-method-guide.webp\" alt=\"Heating a hydrated salt in a crucible to remove water of crystallisation\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n        <\/div>\n        <div class=\"ols-zoom-card-caption\">\n          <p>Heating a hydrated salt in a crucible allows the mass of water lost to be compared with the mass of anhydrous salt remaining.<\/p>\n        <\/div>\n      <\/div>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>Finding x in a Hydrated Salt Formula<\/h2><\/div>\n        <p>To find the value of <strong>x<\/strong> in a formula such as ZnSO<sub>4<\/sub>\u00b7xH<sub>2<\/sub>O, compare the number of moles of anhydrous salt with the number of moles of water lost.<\/p>\n        <ol>\n          <li>Find the mass of water lost: hydrated salt mass minus anhydrous salt mass.<\/li>\n          <li>Calculate moles of anhydrous salt using mass \u00f7 M<sub>r<\/sub>.<\/li>\n          <li>Calculate moles of water using mass \u00f7 18.0.<\/li>\n          <li>Divide both mole values by the smaller value to find the simplest ratio.<\/li>\n        <\/ol>\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>Worked pattern<\/h3>\n            <p>If 3.51 g of hydrated zinc sulfate gives 1.97 g of anhydrous ZnSO<sub>4<\/sub>, then mass of water lost = 3.51 \u2212 1.97 = 1.54 g.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>Mole ratio<\/h3>\n            <p>Moles of ZnSO<sub>4<\/sub> = 1.97 \u00f7 161.5 = 0.0122. Moles of H<sub>2<\/sub>O = 1.54 \u00f7 18.0 = 0.0856. The ratio is approximately 1:7, so x = <strong>7<\/strong>.<\/p>\n          <\/div>\n        <\/div>\n        <div class=\"ols-key-box\"><p><strong>Final formula:<\/strong> ZnSO<sub>4<\/sub>\u00b77H<sub>2<\/sub>O.<\/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: Order the Steps to Find x<\/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-625\" class=\"h5p-iframe\" data-content-id=\"625\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrated Salts Put in Order: Finding x for Barium Chloride\"><\/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: Find x from Experimental Data<\/h2>\n<p>Work each problem out on paper before you turn the card.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-292\" class=\"h5p-iframe\" data-content-id=\"292\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrated Salts Flip Cards: Finding x from Heating Data\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Common Exam Points<\/h2><\/div>\n        <ul>\n          <li>Relative atomic mass, A<sub>r<\/sub>, is measured relative to 1\/12 of the mass of a carbon-12 atom.<\/li>\n          <li>A<sub>r<\/sub> is a weighted mean when an element has naturally occurring isotopes.<\/li>\n          <li>Relative formula mass, M<sub>r<\/sub>, is found by adding the A<sub>r<\/sub> values of all atoms in the formula.<\/li>\n          <li>Use brackets carefully. A subscript outside a bracket multiplies every atom inside the bracket.<\/li>\n          <li>Percentage composition uses the mass contribution of one element divided by the total M<sub>r<\/sub>, multiplied by 100.<\/li>\n          <li>Hydrated salts contain water of crystallisation.<\/li>\n          <li>Anhydrous salts contain no water of crystallisation.<\/li>\n          <li>In hydrated salt calculations, the mass of water lost is found by subtracting the anhydrous mass from the hydrated mass.<\/li>\n          <li>To find x in a hydrate, compare the mole ratio of anhydrous salt to water.<\/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>A<sub>r<\/sub>:<\/strong> weighted mean mass of atoms compared with 1\/12 of carbon-12.<\/li>\n          <li><strong>M<sub>r<\/sub>:<\/strong> sum of all relative atomic masses in the formula.<\/li>\n          <li><strong>Percentage composition:<\/strong> mass of the element in the formula \u00f7 total M<sub>r<\/sub> \u00d7 100.<\/li>\n          <li><strong>Hydrated salt:<\/strong> a crystalline salt containing water of crystallisation.<\/li>\n          <li><strong>Anhydrous salt:<\/strong> the salt after water of crystallisation has been removed.<\/li>\n          <li><strong>Hydrate ratio:<\/strong> calculate moles of salt and water, then divide by the smaller value.<\/li>\n        <\/ul>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxComparingMasses001\" 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 decoding=\"async\" class=\"ols-image-lightbox-img\" src=\"\" alt=\"\">\n        <\/div>\n      <\/div>\n\n      <script>\n        (function(){\n          var page = document.querySelector(\".ols-comparing-masses-page\");\n          if (!page) {\n            return;\n          }\n\n          var lightbox = page.querySelector(\"#olsImageLightboxComparingMasses001\");\n          if (!lightbox) {\n            return;\n          }\n\n          var lightboxImage = lightbox.querySelector(\".ols-image-lightbox-img\");\n          var closeButton = lightbox.querySelector(\".ols-image-lightbox-close\");\n          var clickableImages = page.querySelectorAll(\"img.ols-lightbox-target\");\n\n          function openLightbox(image) {\n            lightboxImage.src = image.currentSrc || image.src;\n            lightboxImage.alt = image.alt || \"Expanded revision image\";\n            lightbox.classList.add(\"is-open\");\n            lightbox.setAttribute(\"aria-hidden\", \"false\");\n\n            if (!document.body.dataset.olsPreviousOverflow) {\n              document.body.dataset.olsPreviousOverflow = document.body.style.overflow || \"default\";\n            }\n\n            document.body.style.overflow = \"hidden\";\n          }\n\n          function closeLightbox() {\n            lightbox.classList.remove(\"is-open\");\n            lightbox.setAttribute(\"aria-hidden\", \"true\");\n            lightboxImage.src = \"\";\n\n            if (document.body.dataset.olsPreviousOverflow) {\n              document.body.style.overflow = document.body.dataset.olsPreviousOverflow === \"default\" ? 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M<sub>r<\/sub> is used for a formula and is found by adding together the A<sub>r<\/sub> values for all atoms in that formula.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why is relative formula mass better than relative molecular mass?<\/h3>\n            <p>Relative molecular mass applies properly to molecular substances. Relative formula mass is more general because it can be used for ionic compounds, molecules and giant structures.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>How do I calculate percentage composition?<\/h3>\n            <p>Find the total mass contribution from the element in the formula, divide by the M<sub>r<\/sub> of the compound, then multiply by 100.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What is water of crystallisation?<\/h3>\n            <p>Water of crystallisation is water chemically incorporated into the crystal structure of a hydrated salt in a fixed ratio.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>How do I find x in a hydrated salt formula?<\/h3>\n            <p>Calculate moles of anhydrous salt and moles of water lost, then divide both values by the smaller mole value to obtain the simplest ratio.<\/p>\n          <\/div>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Build the surrounding mole calculation skills needed for formulae, equations and amounts of substance questions.<\/p>\n        <div class=\"ols-related-grid\">\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/mole-calculations\/calculations-involving-moles\/\">Calculations Involving Moles<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/empirical-and-molecular-formulae\/molecular-formulae\/\">Molecular Formulae<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/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\/cie\/topic-2-atoms-molecules-and-stoichiometry\/\">Topic 2 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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