{"id":5279,"date":"2026-06-19T04:55:11","date_gmt":"2026-06-19T03:55:11","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/writing-chemical-equations\/"},"modified":"2026-09-19T20:35:20","modified_gmt":"2026-09-19T19:35:20","slug":"writing-chemical-equations","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/writing-chemical-equations\/","title":{"rendered":"Writing Chemical Equations"},"content":{"rendered":"\n<!--\n===============================================================================\nONLINE LEARNING SYSTEM COPYRIGHT NOTICE\n\u00a9 Online Learning System. 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Substance<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/relative-masses\/\">Relative Masses<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/the-mole-and-avogadro-constant\/\">The Mole and Avogadro Constant<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/empirical-and-molecular-formulae\/\">Empirical and Molecular Formulae<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/empirical-and-molecular-formulae\/empirical-formula\/\">Empirical Formula<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/empirical-and-molecular-formulae\/molecular-formula\/\">Molecular Formula<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/\">Chemical Equations and Reacting Masses<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/writing-chemical-equations\/\">Writing Chemical Equations<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/ionic-equations\/\">Ionic Equations<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/calculations-using-reacting-masses\/\">Calculations Using Reacting Masses<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/concentration-and-titration-calculations\/\">Concentration and Titration Calculations<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/concentration-and-titration-calculations\/concentrations-of-solutions\/\">Concentrations of Solutions<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/concentration-and-titration-calculations\/titration-calculations\/\">Titration Calculations<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/gas-volumes-and-the-ideal-gas-equation\/\">Gas Volumes and the Ideal Gas Equation<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/percentage-yield\/\">Percentage Yield<\/a>\r\n      <\/li>\r\n\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/atom-economy\/\">Atom Economy<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Adjacent Topics<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-1-atomicstructure\/\">3.1.1 Atomic Structure<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-3-bonding\/\">3.1.3 Bonding<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-4-energetics\/\">3.1.4 Energetics<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-2-1-periodicity\/\">3.2.1 Periodicity<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n<\/aside>\r\n\r\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 = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    sidebar.querySelectorAll('.active, .active-main, .parent-active').forEach(function(item) {\r\n      item.classList.remove('active', 'active-main', 'parent-active');\r\n    });\r\n\r\n    sidebar.querySelectorAll('a[data-ols-disabled-parent=\"1\"], 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true;\r\n  }\r\n\r\n  if (!highlightActive()) {\r\n    document.addEventListener('DOMContentLoaded', highlightActive);\r\n  }\r\n})();\r\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\/aqa\/\">AQA<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/\">3.1.2 Amount of Substance<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/\">Chemical Equations and Reacting Masses<\/a> \/\n        <span>Writing Chemical Equations<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Writing Chemical Equations<\/h1>\n        <p class=\"ols-page-intro\">A concise AQA A Level Chemistry revision guide to writing and balancing chemical equations, including formulae, subscripts, coefficients, state symbols and atom conservation.<\/p>\n\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Paper 1 and Paper 2<\/div>\n          <div class=\"ols-badge\">AQA<\/div>\n          <div class=\"ols-badge\">3.1.2 Amount of Substance<\/div>\n          <div class=\"ols-badge\">7405\/1 and 7405\/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: Conservation of Mass<\/h2>\n<p>Before you start, check the two GCSE ideas that balancing equations is built on.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"630\"><\/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>What a Chemical Equation Shows<\/h2><\/div>\n        <p>A <strong>chemical equation<\/strong> represents a chemical reaction using formulae and symbols. The substances that react are written on the left-hand side, and the substances formed are written on the right-hand side.<\/p>\n        <p>The reaction arrow means <strong>reacts to form<\/strong>. For example, when hydrogen reacts with oxygen, water is made:<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Example:<\/strong> 2H<sub>2<\/sub> + O<sub>2<\/sub> \u2192 2H<sub>2<\/sub>O<\/p><\/div>\n        <p>When writing equations, the aim is to show the correct substances and the correct number of each type of atom on both sides.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> atoms are rearranged during a chemical reaction. They are not created or destroyed.<\/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>What Small Subscript Numbers Mean<\/h2><\/div>\n        <p>The small subscript number in a chemical formula tells you how many atoms of the element immediately before it are present in one formula unit or molecule.<\/p>\n        <p>For example, in <strong>H<sub>2<\/sub>SO<sub>4<\/sub><\/strong>, the 2 applies only to hydrogen and the 4 applies only to oxygen. There is no small number after sulfur, so there is one sulfur atom.<\/p>\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Formula<\/th>\n                <th>Atom count<\/th>\n                <th>Meaning<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td data-label=\"Formula\">H<sub>2<\/sub>SO<sub>4<\/sub><\/td>\n                <td data-label=\"Atom count\">2 H, 1 S, 4 O<\/td>\n                <td data-label=\"Meaning\">The 2 applies to H only, and the 4 applies to O only.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Formula\">CH<sub>4<\/sub><\/td>\n                <td data-label=\"Atom count\">1 C, 4 H<\/td>\n                <td data-label=\"Meaning\">The 4 applies to hydrogen only.<\/td>\n              <\/tr>\n              <tr>\n                <td data-label=\"Formula\">Ca(OH)<sub>2<\/sub><\/td>\n                <td data-label=\"Atom count\">1 Ca, 2 O, 2 H<\/td>\n                <td data-label=\"Meaning\">The 2 outside the brackets applies to everything inside the brackets.<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\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-formulas-and-atom-counting-guide.webp\" alt=\"Chemistry formula and atom counting guide showing subscripts, coefficients and bracketed groups\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n          <\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>Use the position of each number carefully. A small number inside a formula applies only to the atom or bracketed group immediately before it.<\/p>\n          <\/div>\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>What Big Numbers in Front Mean<\/h2><\/div>\n        <p>A large number placed in front of a formula is called a <strong>coefficient<\/strong>. It multiplies the whole formula.<\/p>\n        <p>For example, <strong>5CH<sub>4<\/sub><\/strong> means five molecules of CH<sub>4<\/sub>. Each CH<sub>4<\/sub> contains one carbon atom and four hydrogen atoms, so 5CH<sub>4<\/sub> contains <strong>5 carbon atoms<\/strong> and <strong>20 hydrogen atoms<\/strong>.<\/p>\n        <div class=\"ols-rule-list\">\n          <div class=\"ols-rule-item\">\n            <h3>2H<sub>2<\/sub>O<\/h3>\n            <p>Two water molecules. Total atoms: 4 H and 2 O.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>3CO<sub>2<\/sub><\/h3>\n            <p>Three carbon dioxide molecules. Total atoms: 3 C and 6 O.<\/p>\n          <\/div>\n          <div class=\"ols-rule-item\">\n            <h3>4Al<sub>2<\/sub>O<sub>3<\/sub><\/h3>\n            <p>Four formula units of aluminium oxide. Total atoms: 8 Al and 12 O.<\/p>\n          <\/div>\n        <\/div>\n        <div class=\"ols-key-box\"><p><strong>Important rule:<\/strong> you may change coefficients when balancing an equation, but you must not alter the small subscript numbers inside a correct formula.<\/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: Counting Atoms in Formulae<\/h2>\n<p>Use subscripts, brackets and the number in front to count atoms in formulae you have not seen on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-631\" class=\"h5p-iframe\" data-content-id=\"631\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Chemical Formulae Flashcards: Counting Atoms with Subscripts, Brackets and Coefficients\"><\/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>Why Equations Must Be Balanced<\/h2><\/div>\n        <p>A balanced equation has the same number of each type of atom on both sides. This follows the conservation of mass: atoms are not lost, gained or changed into different atoms during chemical reactions.<\/p>\n        <p>Balancing an equation means adjusting the coefficients in front of formulae until the atom counts match on the left-hand side and right-hand side.<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Balanced equation:<\/strong> an equation in which the number of atoms of each element is the same on both sides of the reaction arrow.<\/p><\/div>\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\/1-Balancing-chemical-equations-made-easy.webp\" alt=\"Balancing chemical equations made easy using coefficients and atom counts\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n          <\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>Balancing is a counting process. The formulae stay fixed, while the numbers in front are adjusted to make the atom counts equal.<\/p>\n          <\/div>\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: Which Change Is Allowed?<\/h2>\n<p>All four equations have matching atom counts, but only one is an acceptable balanced equation.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-632\" class=\"h5p-iframe\" data-content-id=\"632\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Balancing Equations MCQ: Which Change Is Allowed?\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">5<\/div><h2>A Systematic Method for Balancing<\/h2><\/div>\n        <p>Balance equations methodically. Work across the equation and check one element at a time. If an element appears in several places, leave it until later because changing one coefficient may affect more than one atom count.<\/p>\n        <ol>\n          <li>Write the correct formulae for the reactants and products.<\/li>\n          <li>Count the atoms of each element on both sides.<\/li>\n          <li>Add coefficients in front of formulae to balance one element or unchanged group at a time.<\/li>\n          <li>Keep bracketed groups together if the group is unchanged on both sides.<\/li>\n          <li>Add state symbols where required: (s), (l), (g) and (aq).<\/li>\n          <li>Check every atom count again at the end.<\/li>\n        <\/ol>\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\/3-Balancing-chemical-equations-guide.webp\" alt=\"Step-by-step guide for balancing chemical equations\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n          <\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>A systematic approach reduces errors, especially when several elements or bracketed groups are present.<\/p>\n          <\/div>\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: Balanced or Not?<\/h2>\n<p>Count atoms on both sides and click every equation that is already balanced.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-633\" class=\"h5p-iframe\" data-content-id=\"633\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Balancing Equations Mark the Words: Find the Balanced Equations\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">6<\/div><h2>Worked Example: Balancing an Equation<\/h2><\/div>\n        <p>Consider the reaction between magnesium and oxygen:<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Unbalanced:<\/strong> Mg + O<sub>2<\/sub> \u2192 MgO<\/p><\/div>\n        <p>There are two oxygen atoms on the left because oxygen exists as O<sub>2<\/sub>. There is only one oxygen atom in MgO on the right. Place a 2 in front of MgO:<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Step:<\/strong> Mg + O<sub>2<\/sub> \u2192 2MgO<\/p><\/div>\n        <p>This now gives two magnesium atoms on the right, so place a 2 in front of Mg:<\/p>\n        <div class=\"ols-definition-box\"><p><strong>Balanced:<\/strong> 2Mg + O<sub>2<\/sub> \u2192 2MgO<\/p><\/div>\n        <div class=\"ols-key-box\"><p><strong>Check:<\/strong> left side = 2 Mg and 2 O. Right side = 2 Mg and 2 O. The equation is balanced.<\/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: Type the Coefficients<\/h2>\n<p>Balance five new equations by typing the number that goes in front of each formula.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-286\" class=\"h5p-iframe\" data-content-id=\"286\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Balancing Equations Fill in the Blanks: Type the Coefficients\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">7<\/div><h2>Using Bracketed Groups Efficiently<\/h2><\/div>\n        <p>If a group of atoms remains unchanged on both sides of an equation, it is often quicker to count the whole group rather than each individual atom inside it.<\/p>\n        <p>For example, if the sulfate group, SO<sub>4<\/sub><sup>2-<\/sup>, appears unchanged on both sides, count sulfate groups first. This saves time and reduces the chance of losing track of sulfur and oxygen atoms separately.<\/p>\n        <div class=\"ols-key-box\"><p><strong>Exam point:<\/strong> bracketed groups are useful when the same group appears unchanged on both sides. If the group changes, count the individual atoms instead.<\/p><\/div>\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\/2-Balancing-chemical-equations-guide.webp\" alt=\"Balancing chemical equations guide showing atom counting and unchanged groups\" class=\"ols-zoomable-img ols-lightbox-target\" draggable=\"false\">\n          <\/div>\n          <div class=\"ols-zoom-card-caption\">\n            <p>Keep unchanged polyatomic groups together while balancing, then complete a final atom-by-atom check.<\/p>\n          <\/div>\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: From Word Equation to Balanced Equation<\/h2>\n<p>Write each balanced symbol equation on paper, then flip the card to check your formulae and numbers.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-287\" class=\"h5p-iframe\" data-content-id=\"287\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Balanced Symbol Equations: Word Equation Practice\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\"><div class=\"ols-note-icon\">8<\/div><h2>Common Exam Points<\/h2><\/div>\n        <ul>\n          <li>Small subscript numbers are part of the formula and must not be changed when balancing.<\/li>\n          <li>A coefficient in front of a formula multiplies the whole formula.<\/li>\n          <li>If there is no small subscript number after a symbol, there is one atom of that element in the formula.<\/li>\n          <li>Brackets multiply everything inside them. For example, Ca(OH)<sub>2<\/sub> contains two oxygen atoms and two hydrogen atoms.<\/li>\n          <li>Only coefficients are changed during balancing.<\/li>\n          <li>Balance one element at a time, then check the whole equation again at the end.<\/li>\n          <li>Leave elements that appear in several formulae until later where possible.<\/li>\n          <li>Count unchanged groups, such as sulfate or nitrate, as whole groups when this is quicker.<\/li>\n          <li>A balanced equation must have the same number of each type of atom on both sides.<\/li>\n          <li>Use state symbols where required: (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution.<\/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>Subscripts:<\/strong> small numbers inside a formula show how many atoms of the element immediately before them are present.<\/li>\n          <li><strong>Coefficients:<\/strong> big numbers in front of formulae multiply the whole formula.<\/li>\n          <li><strong>Do not alter formulae:<\/strong> never change a small subscript to balance an equation.<\/li>\n          <li><strong>Balance with coefficients:<\/strong> write numbers in front of formulae until the atoms match on both sides.<\/li>\n          <li><strong>Bracketed groups:<\/strong> a number outside brackets multiplies everything inside the brackets.<\/li>\n          <li><strong>Final check:<\/strong> recount every element at the end to make sure no earlier balance has been disrupted.<\/li>\n          <li><strong>State symbols:<\/strong> use (s), (l), (g) and (aq) where required.<\/li>\n        <\/ul>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxWritingChemicalEquations001\" aria-hidden=\"true\" role=\"dialog\" 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ease-in-out infinite;\r\n      transition:\r\n        transform 0.25s ease,\r\n        background 0.25s ease,\r\n        color 0.25s ease;\r\n    }\r\n\r\n    .ols-course-cta-aqa-amount-substance .ols-animation-start-overlay:hover .ols-animation-play-circle {\r\n      transform: scale(1.08);\r\n      background: var(--navy);\r\n      color: #ffffff;\r\n    }\r\n\r\n    .ols-course-cta-aqa-amount-substance .ols-animation-play-icon {\r\n      width: 0;\r\n      height: 0;\r\n      margin-left: 7px;\r\n      border-top: 18px solid transparent;\r\n      border-bottom: 18px solid transparent;\r\n      border-left: 28px solid currentColor;\r\n    }\r\n\r\n    .ols-course-cta-aqa-amount-substance .ols-animation-start-text {\r\n      max-width: 540px;\r\n      margin: 0;\r\n      color: #ffffff;\r\n      font-size: clamp(20px, 3vw, 32px);\r\n      line-height: 1.2;\r\n      font-weight: 800;\r\n      text-shadow: 0 8px 20px rgba(0, 0, 0, 0.28);\r\n    }\r\n\r\n    .ols-course-cta-aqa-amount-substance .ols-animation-pause-button {\r\n      position: absolute;\r\n      left: 16px;\r\n      bottom: 16px;\r\n      z-index: 7;\r\n      display: none;\r\n      align-items: center;\r\n      justify-content: center;\r\n      min-width: 92px;\r\n      padding: 10px 16px;\r\n      border: 0;\r\n      border-radius: 999px;\r\n      background: rgba(28, 36, 75, 0.92);\r\n      color: #ffffff;\r\n      font-family: Poppins, Arial, sans-serif;\r\n      font-size: 14px;\r\n      line-height: 1.2;\r\n      font-weight: 800;\r\n      cursor: pointer;\r\n      box-shadow: 0 12px 28px rgba(28, 36, 75, 0.24);\r\n      transition:\r\n        transform 0.2s ease,\r\n        background 0.2s ease,\r\n        box-shadow 0.2s ease;\r\n    }\r\n\r\n    .ols-course-cta-aqa-amount-substance .ols-animation-pause-button:hover {\r\n      transform: translateY(-2px);\r\n      background: var(--blue);\r\n      box-shadow: 0 16px 34px rgba(37, 99, 235, 0.28);\r\n    }\r\n\r\n    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flex-shrink: 0;\r\n      padding: 12px 22px;\r\n      font-size: 15px;\r\n    }\r\n\r\n    @media (max-width: 900px) {\r\n      .ols-course-cta-aqa-amount-substance .ols-course-cta-top {\r\n        grid-template-columns: 1fr;\r\n      }\r\n\r\n      .ols-course-cta-aqa-amount-substance .ols-course-cta-image-link {\r\n        max-width: 520px;\r\n        margin: 0 auto;\r\n      }\r\n\r\n      .ols-course-cta-aqa-amount-substance .ols-course-cta-intro-stats {\r\n        grid-template-columns: 1fr;\r\n      }\r\n\r\n      .ols-course-cta-aqa-amount-substance .ols-animation-frame-shell {\r\n        height: 520px;\r\n      }\r\n    }\r\n\r\n    @media (max-width: 760px) {\r\n      .ols-course-cta-aqa-amount-substance {\r\n        margin: 26px 0 22px;\r\n      }\r\n\r\n      .ols-course-cta-aqa-amount-substance .ols-course-cta-card {\r\n        padding: 20px;\r\n        border-radius: 24px;\r\n      }\r\n\r\n      .ols-course-cta-aqa-amount-substance .ols-course-cta-header-row {\r\n        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.ols-course-cta-aqa-amount-substance .ols-animation-pause-button {\r\n        left: 12px;\r\n        bottom: 12px;\r\n        min-width: 84px;\r\n        padding: 9px 14px;\r\n        font-size: 13px;\r\n      }\r\n\r\n      .ols-course-cta-aqa-amount-substance .ols-course-button {\r\n        width: 100%;\r\n      }\r\n    }\r\n  <\/style>\r\n\r\n  <div class=\"ols-course-cta-card\">\r\n\r\n    <div class=\"ols-course-cta-top\">\r\n\r\n      <a\r\n        class=\"ols-course-cta-image-link\"\r\n        href=\"[insert here]\"\r\n        aria-label=\"Open the AQA A Level Chemistry 3.1.2 Amount of Substance course page\">\r\n\r\n        <div class=\"ols-course-cta-image\">\r\n          <img decoding=\"async\"\r\n            src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/3-1-2-Amount-of-Substance.jpg\"\r\n            alt=\"AQA A Level Chemistry 3.1.2 Amount of Substance interactive course banner\"\r\n          >\r\n        <\/div>\r\n\r\n      <\/a>\r\n\r\n      <div class=\"ols-course-cta-content\">\r\n\r\n        <div class=\"ols-course-cta-header-row\">\r\n\r\n          <div class=\"ols-course-cta-kicker\">AQA 7405<\/div>\r\n          <div class=\"ols-course-cta-kicker\">Paper 1 &amp; Paper 2<\/div>\r\n          <div class=\"ols-course-cta-kicker\">3.1.2 Amount of Substance<\/div>\r\n\r\n        <\/div>\r\n\r\n        <h2>\r\n          Master Amount of Substance for AQA A Level Chemistry\r\n        <\/h2>\r\n\r\n        <div class=\"ols-course-cta-heading-button\">\r\n          <a\r\n            class=\"ols-course-button ols-course-button-top\"\r\n            href=\"[insert here]\">\r\n            View Course\r\n          <\/a>\r\n        <\/div>\r\n\r\n      <\/div>\r\n\r\n    <\/div>\r\n\r\n    <div class=\"ols-course-cta-intro-stats\">\r\n\r\n      <p class=\"ols-course-cta-intro\">\r\n        Continue from these free revision notes into the full 3.1.2 Amount of Substance course, covering moles, Avogadro constant, empirical and molecular formulae, reacting masses, concentration, titrations, gas volumes, percentage yield and atom economy with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a personalised progress report.\r\n      <\/p>\r\n\r\n      <div class=\"ols-course-cta-stats\">\r\n\r\n        <div class=\"ols-course-stat\">\r\n          <span>Guided learning<\/span>\r\n          <strong>Coming soon<\/strong>\r\n        <\/div>\r\n\r\n        <div class=\"ols-course-stat\">\r\n          <span>Video lessons<\/span>\r\n          <strong>Coming soon<\/strong>\r\n        <\/div>\r\n\r\n        <div class=\"ols-course-stat\">\r\n          <span>MCQ practice<\/span>\r\n          <strong>Coming soon<\/strong>\r\n        <\/div>\r\n\r\n        <div class=\"ols-course-stat\">\r\n          <span>SAQ practice<\/span>\r\n          <strong>Coming soon<\/strong>\r\n        <\/div>\r\n\r\n      <\/div>\r\n\r\n    <\/div>\r\n\r\n    <div class=\"ols-course-cta-features\">\r\n\r\n      <div class=\"ols-course-feature\">\r\n        <h3>Guided video teaching<\/h3>\r\n        <p>\r\n          Learn the chemistry and exam technique through structured video lessons with worked examples and walkthroughs.\r\n        <\/p>\r\n      <\/div>\r\n\r\n      <div class=\"ols-course-feature\">\r\n        <h3>Instant MCQ feedback<\/h3>\r\n        <p>\r\n          Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.\r\n        <\/p>\r\n      <\/div>\r\n\r\n      <div class=\"ols-course-feature\">\r\n        <h3>Teacher-marked SAQs<\/h3>\r\n        <p>\r\n          Submit written exam responses and receive chemistry specialist feedback with improvement guidance.\r\n        <\/p>\r\n      <\/div>\r\n\r\n      <div class=\"ols-course-feature\">\r\n        <h3>Progress tracking<\/h3>\r\n        <p>\r\n          Identify strengths and weaknesses across the full 3.1.2 Amount of Substance specification, including mole calculations, formulae, solution calculations, gas calculations, yield and atom economy.\r\n        <\/p>\r\n      <\/div>\r\n\r\n    <\/div>\r\n\r\n    <div class=\"ols-course-animation-wrap\">\r\n\r\n      <h3 class=\"ols-course-animation-title\">\r\n        See how the course works\r\n      <\/h3>\r\n\r\n      <div class=\"ols-animation-frame-shell\">\r\n        <iframe\r\n          id=\"olsCourseAnimationFrame312\"\r\n          title=\"OLS course preview animation\"\r\n          loading=\"lazy\"\r\n          referrerpolicy=\"no-referrer\">\r\n        <\/iframe>\r\n\r\n        <button\r\n          class=\"ols-animation-start-overlay\"\r\n          id=\"olsCourseAnimationStart312\"\r\n          type=\"button\"\r\n          aria-label=\"Play OLS course preview animation\">\r\n          <span class=\"ols-animation-start-content\">\r\n            <span class=\"ols-animation-play-circle\" aria-hidden=\"true\">\r\n              <span class=\"ols-animation-play-icon\"><\/span>\r\n            <\/span>\r\n            <span class=\"ols-animation-start-text\">\r\n              Play course preview animation\r\n            <\/span>\r\n          <\/span>\r\n        <\/button>\r\n\r\n        <button\r\n          class=\"ols-animation-pause-button\"\r\n          id=\"olsCourseAnimationPause312\"\r\n          type=\"button\"\r\n          aria-label=\"Pause course preview animation\">\r\n          Pause\r\n        <\/button>\r\n      <\/div>\r\n\r\n      <p class=\"ols-course-video-status\">\r\n        Click play to start the course preview animation.\r\n      <\/p>\r\n\r\n    <\/div>\r\n\r\n    <div class=\"ols-course-cta-bottom\">\r\n\r\n      <a\r\n        class=\"ols-course-button\"\r\n        href=\"[insert here]\">\r\n        View Course\r\n      <\/a>\r\n\r\n    <\/div>\r\n\r\n  <\/div>\r\n\r\n  <template id=\"olsCourseAnimationTemplate312\">\r\n<!DOCTYPE html>\r\n<html lang=\"en\">\r\n<head>\r\n<meta charset=\"UTF-8\">\r\n<meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0\">\r\n<title>OLS Amount of Substance Promo Animation<\/title>\r\n\r\n<style>\r\n*{box-sizing:border-box;}\r\n\r\nhtml.ols-animation-paused *,\r\nhtml.ols-animation-paused *::before,\r\nhtml.ols-animation-paused *::after{\r\n  animation-play-state:paused!important;\r\n}\r\n\r\n:root{\r\n  --ols-video-screen-w:1180;\r\n  --ols-video-screen-h:664;\r\n  --ols-video-screen-scale:1;\r\n  --ols-mcq-screen-w:1360;\r\n  --ols-mcq-screen-h:1130;\r\n  --ols-mcq-screen-scale:1;\r\n  --ols-saq-screen-w:1360;\r\n  --ols-saq-screen-h:965;\r\n  --ols-saq-screen-scale:1;\r\n}\r\n\r\nhtml,\r\nbody{\r\n  width:100%;\r\n  height:100%;\r\n}\r\n\r\nbody{\r\n  margin:0;\r\n  overflow:hidden;\r\n  font-family:Poppins,Arial,sans-serif;\r\n  background:#e9efff;\r\n}\r\n\r\n.ols-combined-stage{\r\n  position:relative;\r\n  width:100vw;\r\n  height:100vh;\r\n  overflow:hidden;\r\n  background:\r\n    radial-gradient(circle at top left,rgba(70,127,247,0.14),transparent 34%),\r\n    linear-gradient(135deg,#f8fbff 0%,#e9efff 100%);\r\n}\r\n\r\n.ols-scene{\r\n  position:absolute;\r\n  inset:0;\r\n  width:100vw;\r\n  height:100vh;\r\n  opacity:0;\r\n  visibility:hidden;\r\n  pointer-events:none;\r\n}\r\n\r\n.ols-scene.active{\r\n  opacity:1;\r\n  visibility:visible;\r\n  pointer-events:auto;\r\n}\r\n\r\n.ols-scene.fade-out{\r\n  animation:olsSceneFadeOut 0.85s ease forwards;\r\n}\r\n\r\n@keyframes olsSceneFadeOut{\r\n  to{\r\n    opacity:0;\r\n    visibility:hidden;\r\n  }\r\n}\r\n\r\n.ols-title-cursor{\r\n  display:inline-block;\r\n  width:5px;\r\n  height:0.85em;\r\n  background:#1C244B;\r\n  margin-left:8px;\r\n  transform:translateY(8px);\r\n  animation:olsBlink 0.8s infinite;\r\n}\r\n\r\n.cursor{\r\n  display:inline-block;\r\n  width:3px;\r\n  height:28px;\r\n  background:#1c244b;\r\n  margin-left:3px;\r\n  transform:translateY(5px);\r\n  animation:olsBlink 0.8s infinite;\r\n}\r\n\r\n@keyframes olsBlink{\r\n  0%,45%{opacity:1;}\r\n  46%,100%{opacity:0;}\r\n}\r\n\r\n.ols-video-stage,\r\n.ols-mcq-stage,\r\n.ols-saq-stage{\r\n  width:100vw;\r\n  height:100vh;\r\n  display:flex;\r\n  justify-content:center;\r\n  align-items:center;\r\n  padding:0;\r\n  overflow:hidden;\r\n  background:\r\n    radial-gradient(circle at top left,rgba(70,127,247,0.14),transparent 34%),\r\n    linear-gradient(135deg,#f8fbff 0%,#e9efff 100%);\r\n}\r\n\r\n.ols-video-title-screen,\r\n.ols-mcq-intro-screen,\r\n.ols-saq-intro{\r\n  position:absolute;\r\n  inset:0;\r\n  z-index:50;\r\n  display:flex;\r\n  justify-content:center;\r\n  align-items:center;\r\n  background:\r\n    radial-gradient(circle at top left,rgba(70,127,247,0.14),transparent 34%),\r\n    linear-gradient(135deg,#f8fbff 0%,#e9efff 100%);\r\n  opacity:1;\r\n  visibility:visible;\r\n  pointer-events:none;\r\n}\r\n\r\n.ols-video-title-screen.hide{animation:olsVideoTitleFadeOut 0.85s ease forwards;}\r\n.ols-mcq-intro-screen.hide{animation:olsMcqIntroFadeOut 0.85s ease forwards;}\r\n.ols-saq-intro.hide{animation:olsSaqIntroFadeOut 0.85s ease forwards;}\r\n\r\n@keyframes olsVideoTitleFadeOut{to{opacity:0;visibility:hidden;}}\r\n@keyframes olsMcqIntroFadeOut{to{opacity:0;visibility:hidden;}}\r\n@keyframes olsSaqIntroFadeOut{to{opacity:0;visibility:hidden;}}\r\n\r\n.ols-video-title-wrap,\r\n.ols-mcq-intro-title-wrap,\r\n.ols-saq-intro-title-wrap{\r\n  max-width:1250px;\r\n  padding:0 34px;\r\n  text-align:center;\r\n}\r\n\r\n.ols-video-title,\r\n.ols-mcq-intro-title,\r\n.ols-saq-intro-title{\r\n  margin:0;\r\n  font-family:Poppins,Arial,sans-serif;\r\n  font-size:clamp(52px,8vw,124px);\r\n  font-weight:600;\r\n  line-height:1.08;\r\n  color:#1C244B;\r\n  text-shadow:-9px 0 9px rgba(28,36,75,0.16);\r\n  letter-spacing:-0.045em;\r\n}\r\n\r\n#videoTitleText,\r\n#mcqIntroTitleText,\r\n#saqIntroTitleText{\r\n  white-space:pre-wrap;\r\n}\r\n\r\n.ols-video-scene{\r\n  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28px;\r\n  font-size:24px;\r\n  line-height:1.35;\r\n  color:#111827;\r\n}\r\n\r\n.mcq-options-wrap{\r\n  display:flex;\r\n  flex-direction:column;\r\n  gap:17px;\r\n  max-width:1200px;\r\n}\r\n\r\n.mcq-option-row{\r\n  display:flex;\r\n  align-items:center;\r\n  min-height:34px;\r\n  position:relative;\r\n}\r\n\r\n.mcq-radio{\r\n  width:28px;\r\n  height:28px;\r\n  border-radius:50%;\r\n  border:2px solid #6b7280;\r\n  margin-right:16px;\r\n  background:transparent;\r\n  position:relative;\r\n  flex:0 0 auto;\r\n}\r\n\r\n.mcq-radio::after{\r\n  content:\"\";\r\n  position:absolute;\r\n  inset:5px;\r\n  border-radius:50%;\r\n  background:#6b7280;\r\n  opacity:0;\r\n  transform:scale(0.4);\r\n  transition:opacity 0.25s ease,transform 0.25s ease;\r\n}\r\n\r\n.mcq-option-row.selected .mcq-radio::after{\r\n  opacity:1;\r\n  transform:scale(1);\r\n}\r\n\r\n.mcq-radio-ripple{\r\n  position:absolute;\r\n  left:14px;\r\n  top:50%;\r\n  width:28px;\r\n  height:28px;\r\n  border-radius:50%;\r\n  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forwards;\r\n}\r\n\r\n@keyframes olsSaqScreenEnter{\r\n  from{opacity:0;transform:translateY(24px) scale(0.96);}\r\n  to{opacity:1;transform:translateY(0) scale(1);}\r\n}\r\n\r\n.saq-question-area{\r\n  height:335px;\r\n  background:#eaf0ff;\r\n  padding:30px 40px 32px;\r\n}\r\n\r\n.saq-question-text{\r\n  font-size:25px;\r\n  line-height:1.42;\r\n  color:#111827;\r\n  margin-bottom:22px;\r\n}\r\n\r\n.saq-student-box{\r\n  height:195px;\r\n  background:#f3f4f6;\r\n  border:2px solid #cfd6e3;\r\n  border-radius:8px;\r\n  padding:23px 26px;\r\n  font-size:24px;\r\n  line-height:1.55;\r\n  color:#4b5563;\r\n  position:relative;\r\n  overflow:hidden;\r\n}\r\n\r\n.saq-teacher-feedback{\r\n  height:630px;\r\n  background:#dff3e6;\r\n  color:#075f3b;\r\n  padding:28px 40px 30px;\r\n  font-size:23px;\r\n  line-height:1.47;\r\n  opacity:0;\r\n  transform:translateY(20px);\r\n  transition:opacity 0.7s ease,transform 0.7s ease;\r\n  overflow:hidden;\r\n}\r\n\r\n.saq-teacher-feedback.show{\r\n  opacity:1;\r\n  transform:translateY(0);\r\n}\r\n\r\n#saqTeacherText{\r\n  white-space:pre-wrap;\r\n}\r\n\r\n.saq-teacher-cursor{\r\n  background:#075f3b;\r\n}\r\n\r\n@media(max-width:900px){\r\n  .ols-video-title,\r\n  .ols-mcq-intro-title,\r\n  .ols-saq-intro-title{\r\n    font-size:clamp(42px,11vw,78px);\r\n    line-height:1.12;\r\n  }\r\n\r\n  .ols-title-cursor{\r\n    width:4px;\r\n    margin-left:6px;\r\n  }\r\n\r\n  .ols-video-card{\r\n    border-radius:20px;\r\n  }\r\n}\r\n<\/style>\r\n<\/head>\r\n\r\n<body>\r\n<div class=\"ols-combined-stage\">\r\n\r\n  <section id=\"sceneVideo\" class=\"ols-scene active\">\r\n    <div class=\"ols-video-stage\">\r\n      <div id=\"videoTitleScreen\" class=\"ols-video-title-screen\">\r\n        <div class=\"ols-video-title-wrap\">\r\n          <h1 class=\"ols-video-title\">\r\n            <span id=\"videoTitleText\"><\/span><span id=\"videoTitleCursor\" class=\"ols-title-cursor\"><\/span>\r\n          <\/h1>\r\n        <\/div>\r\n      <\/div>\r\n\r\n      <section id=\"videoScene\" class=\"ols-video-scene\">\r\n        <div class=\"ols-video-scale-wrap\">\r\n          <div class=\"ols-video-card\">\r\n            <video\r\n              id=\"lessonVideo\"\r\n              src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Virtual-Lesson.mp4\"\r\n              muted\r\n              playsinline\r\n              preload=\"auto\">\r\n            <\/video>\r\n          <\/div>\r\n        <\/div>\r\n      <\/section>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section id=\"sceneMcq\" class=\"ols-scene\">\r\n    <div class=\"ols-mcq-stage\">\r\n      <div id=\"mcqIntroScreen\" class=\"ols-mcq-intro-screen\">\r\n        <div class=\"ols-mcq-intro-title-wrap\">\r\n          <h1 class=\"ols-mcq-intro-title\">\r\n            <span id=\"mcqIntroTitleText\"><\/span><span id=\"mcqIntroTitleCursor\" class=\"ols-title-cursor\"><\/span>\r\n          <\/h1>\r\n        <\/div>\r\n      <\/div>\r\n\r\n      <div class=\"ols-mcq-scale-wrap\">\r\n        <main id=\"mcqScreen\" class=\"ols-mcq-screen\">\r\n          <section class=\"mcq-question-area\">\r\n            <p class=\"mcq-question-lead\">A student prepares a sodium hydroxide solution.<\/p>\r\n\r\n            <table class=\"mcq-ionic-table\">\r\n              <thead>\r\n                <tr>\r\n                  <th>Quantity<\/th>\r\n                  <th>Value<\/th>\r\n                <\/tr>\r\n              <\/thead>\r\n              <tbody>\r\n                <tr><td>Concentration of NaOH<\/td><td>0.200 mol dm<sup>\u22123<\/sup><\/td><\/tr>\r\n                <tr><td>Volume used<\/td><td>25.0 cm<sup>3<\/sup><\/td><\/tr>\r\n                <tr><td>Volume in dm<sup>3<\/sup><\/td><td>0.0250 dm<sup>3<\/sup><\/td><\/tr>\r\n              <\/tbody>\r\n            <\/table>\r\n\r\n            <p class=\"mcq-question-main\">What amount of NaOH is present in the 25.0 cm<sup>3<\/sup> sample?<\/p>\r\n\r\n            <div class=\"mcq-options-wrap\">\r\n              <div id=\"mcqWrongOption\" class=\"mcq-option-row\">\r\n                <span class=\"mcq-radio\"><\/span>\r\n                <span class=\"mcq-radio-ripple\"><\/span>\r\n                <span class=\"mcq-option-label\">5.00 mol<\/span>\r\n\r\n                <span id=\"mcqSpecificFeedback\" class=\"mcq-specific-feedback\">\r\n                  <span id=\"mcqCrossIcon\" class=\"mcq-cross-icon\">\u00d7<\/span>\r\n                  <span class=\"mcq-specific-feedback-text\">\r\n                    <span id=\"mcqSpecificText\"><\/span><span id=\"mcqSpecificCursor\" class=\"cursor mcq-specific-cursor\" style=\"display:none;\"><\/span>\r\n                  <\/span>\r\n                <\/span>\r\n              <\/div>\r\n\r\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">0.500 mol<\/span><\/div>\r\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">0.00500 mol<\/span><\/div>\r\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">0.000500 mol<\/span><\/div>\r\n            <\/div>\r\n\r\n            <button id=\"mcqSubmitButton\" class=\"mcq-submit-button\">Submit answer<\/button>\r\n          <\/section>\r\n\r\n          <section id=\"mcqGeneralFeedback\" class=\"mcq-general-feedback\">\r\n            <span id=\"mcqGeneralText\"><\/span><span id=\"mcqGeneralCursor\" class=\"cursor mcq-general-cursor\" style=\"display:none;\"><\/span>\r\n          <\/section>\r\n        <\/main>\r\n      <\/div>\r\n    <\/div>\r\n  <\/section>\r\n\r\n  <section id=\"sceneSaq\" class=\"ols-scene\">\r\n    <div class=\"ols-saq-stage\">\r\n      <div id=\"saqIntro\" class=\"ols-saq-intro\">\r\n        <div class=\"ols-saq-intro-title-wrap\">\r\n          <h1 class=\"ols-saq-intro-title\">\r\n            <span id=\"saqIntroTitleText\"><\/span><span id=\"saqIntroTitleCursor\" class=\"ols-title-cursor\"><\/span>\r\n          <\/h1>\r\n        <\/div>\r\n      <\/div>\r\n\r\n      <div class=\"ols-saq-scale-wrap\">\r\n        <main id=\"saqScreen\" class=\"ols-saq-screen\">\r\n          <section class=\"saq-question-area\">\r\n            <div class=\"saq-question-text\">\r\n              <strong>(ii)<\/strong>&nbsp;&nbsp; Calcium carbonate reacts with hydrochloric acid.<br>\r\n              Calculate the mass of CaCO<sub>3<\/sub> that reacts with 25.0 cm<sup>3<\/sup> of 0.200 mol dm<sup>\u22123<\/sup> HCl.<br>\r\n              CaCO<sub>3<\/sub> + 2HCl \u2192 CaCl<sub>2<\/sub> + H<sub>2<\/sub>O + CO<sub>2<\/sub>\r\n            <\/div>\r\n\r\n            <div class=\"saq-student-box\">\r\n              <span id=\"saqStudentText\"><\/span><span id=\"saqStudentCursor\" 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Changing it would change the substance. Balance equations by changing only the coefficients in front of formulae.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What does a number in front of a formula mean?<\/h3>\n            <p>A coefficient multiplies the whole formula. For example, 3H<sub>2<\/sub>O contains six hydrogen atoms and three oxygen atoms.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>Why does an equation need to be balanced?<\/h3>\n            <p>An equation must be balanced because atoms are conserved in a chemical reaction. The same number of each type of atom must appear on both sides of the reaction arrow.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>When should I count a group of atoms as a whole group?<\/h3>\n            <p>You can count a group such as sulfate, SO<sub>4<\/sub>, or nitrate, NO<sub>3<\/sub>, as a whole group if it appears unchanged on both sides of the equation.<\/p>\n          <\/div>\n          <div class=\"ols-faq-item\">\n            <h3>What should I do if an element appears in several formulae?<\/h3>\n            <p>Leave it until later where possible. Balance simpler elements or unchanged groups first, then return to the element that appears in several places.<\/p>\n          <\/div>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-related-card\">\n        <h2>Related Topics<\/h2>\n        <p>Use these pages next to strengthen the surrounding skills needed for equations and 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\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/\">Chemical Equations and Reacting Masses<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/ionic-equations\/\">Ionic Equations<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/chemical-equations-and-reacting-masses\/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\/aqa\/3-1-2-amount-of-substance\/the-mole-and-avogadro-constant\/\">The Mole and Avogadro Constant<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/relative-masses\/\">Relative Masses<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-2-amount-of-substance\/\">3.1.2 Amount of Substance<\/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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