A common mistake we see is inverting the fraction for percentage yield, placing the theoretical mass in the numerator. The actual yield is always smaller than or equal to the theoretical yield, so your percentage yield should always be 100% or below. If your answer exceeds 100%, the fraction is upside down.<\/p><\/blockquote>\n
Formula for empirical and molecular formula calculations<\/h2>\n
The empirical formula is the simplest whole-number ratio of atoms in a compound. The molecular formula is the actual number of atoms in one molecule, and it is always a whole-number multiple of the empirical formula.<\/p>\n
The multiplier is found using:<\/p>\n
X = Mr \/ empirical formula mass<\/strong><\/p>\nMolecular formula = x x (empirical formula)<\/strong><\/p>\nFor example, if the empirical formula is CH2 with a mass of 14, and the compound has an Mr of 56, then x = 56 \/ 14 = 4. The molecular formula is therefore C4H8.<\/p>\n
Drawing on years of experience at Times Edu, we find that students who practise this calculation type using percentage composition data, rather than given ratios, are far better prepared for the full range of question formats that appear in Paper 4.<\/p><\/blockquote>\n
Formula for energy change calculations<\/h2>\n
The enthalpy change of a reaction can be calculated using bond energy data. This appears in the chemical energetics section of the syllabus and is regularly tested in Paper 4.<\/p>\n
Delta H = Total energy absorbed (bonds broken in reactants) – Total energy released (bonds formed in products)<\/strong><\/p>\nThe sign convention is critical. A negative delta H means the reaction is exothermic; more energy was released forming bonds than was absorbed breaking them. A positive delta H means the reaction is endothermic.<\/p>\n
One critical detail often overlooked is the direction of the subtraction. Students frequently subtract in the wrong order and then misidentify an exothermic reaction as endothermic. Writing the formula explicitly before substituting values eliminates this error.<\/p><\/blockquote>\n
How to build your own IGCSE Chemistry revision formula sheet<\/h2>\n
Since no formula sheet is provided in the exam, building a personalised one during your revision is an effective study technique, provided you then wean yourself off it before exam day.<\/p>\n
Follow this four-step approach:<\/strong><\/p>\n\n- Write all formulas from memory<\/strong> without looking at notes. Use the table in this guide to check your recall immediately after.<\/li>\n
- Group formulas by topic<\/strong> (moles, yield, energy, rates) so that your brain stores them in connected clusters rather than as isolated facts.<\/li>\n
- Attach each formula to a worked example<\/strong> so that the equation is always linked to a calculation process, not just a symbol pattern.<\/li>\n
- Test yourself with past paper questions<\/strong> from Cambridge IGCSE Chemistry 0620 Papers 2 and 4, timing your response to simulate exam conditions.<\/li>\n<\/ol>\n
The goal is to reach a point where writing n = m \/ Mr feels as automatic as writing your own name. That level of recall only comes from repeated retrieval practice, not repeated reading.<\/p><\/blockquote>\n
Common mistakes when memorising Chemistry formulas<\/h2>\n
The following errors appear repeatedly in Cambridge examiner reports for IGCSE Chemistry. Being aware of them before the exam gives you a measurable advantage.<\/p>\n
\n\n\n| Mistake<\/th>\n | Why it happens<\/th>\n | How to fix it<\/th>\n<\/tr>\n |
\n| Inverting percentage yield fraction<\/td>\n | Students confuse numerator and denominator<\/td>\n | Recall: Actual is always smaller than theoretical<\/td>\n<\/tr>\n |
\n| Using cm3 directly in concentration formula<\/td>\n | Volume unit conversion is forgotten under pressure<\/td>\n | Write V(dm3) = V(cm3) \/ 1000 as a mandatory first line<\/td>\n<\/tr>\n |
\n| Using Ar instead of Mr in mole calculation<\/td>\n | Confusion between atomic and formula mass<\/td>\n | Ar is for single elements; Mr is for compounds<\/td>\n<\/tr>\n |
\n| Wrong sign for enthalpy change<\/td>\n | Subtraction order reversed<\/td>\n | Always write delta H = broken – Formed, not the reverse<\/td>\n<\/tr>\n |
\n| Forgetting to multiply cm3 gas volume by 24,000<\/td>\n | Students only remember 24, not 24,000<\/td>\n | Practise both unit versions in every gas volume question<\/td>\n<\/tr>\n |
\n| Skipping the multiplier step in molecular formula<\/td>\n | Students stop at empirical formula<\/td>\n | Always check: Does the question ask for empirical or molecular?<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nFrequently asked questions<\/h2>\nDo you get a formula sheet in IGCSE Chemistry?<\/strong><\/p>\nNo. Cambridge IGCSE Chemistry 0620 does not provide a formula sheet in any exam paper. All quantitative equations must be memorised by the student.<\/p>\n Do you get a Periodic Table in IGCSE Chemistry?<\/strong><\/p>\nYes. A complete Periodic Table, including relative atomic mass (Ar) values, is provided on the back page of Paper 2 and Paper 4. It is not automatically included in Paper 5 or Paper 6.<\/p>\n What formulas do I need to know for IGCSE Chemistry?<\/strong><\/p>\nThe core formulas include: N = m \/ Mr, n = C x V, n = V \/ 24, percentage yield, percentage purity, empirical formula multiplier, delta H from bond energies, and average rate of reaction. All are covered in detail in this guide.<\/p>\n Is molar gas volume given in the IGCSE Chemistry exam?<\/strong><\/p>\nThe value of 24 dm3 per mol at room temperature and pressure (r.t.p.) Is typically stated within the relevant question in Paper 2 and Paper 4. Knowing it in advance allows faster, more confident working.<\/p>\n Do you need to memorise qualitative analysis tests for IGCSE Chemistry?<\/strong><\/p>\nIon tests and gas tests are provided as reference tables in Paper 5 and Paper 6. For Paper 2 and Paper 4, understanding of qualitative analysis is tested through written theory, so you should understand the concepts even if the tables are not provided.<\/p>\n Do you need to memorise mole equations for IGCSE Chemistry?<\/strong><\/p>\nYes. The mole equations are not provided anywhere in the exam and must come from memory. They are the most heavily tested quantitative skills across both theory papers.<\/p>\n What information is provided in Cambridge IGCSE Chemistry exams?<\/strong><\/p>\nThe Periodic Table is provided in Paper 2 and Paper 4. Qualitative analysis reference tables are provided in Paper 5 and Paper 6. Specific numerical data (such as molar gas volume) is stated within individual questions when required. No formula sheet is provided in any paper.<\/p>\n\n\n \n \n \n \n \n \n \n\n <\/div>\n <\/div>\n \n \n\n <\/div>\n <\/div>\n \n \n\n <\/div>\n <\/div>\n \n \n\n <\/div>\n <\/div>\n \n \n\n <\/div>\n <\/div>\n <\/div>\n \n \n \n \n\n <\/div>\n <\/div>\n \n \n\n <\/div>\n <\/div>\n \n \n\n <\/div>\n <\/div>\n \n \n\n <\/div>\n <\/div>\n \n \n\n <\/div>\n <\/div>\n <\/div>\n<\/div>\n \n\n \n 5\/5 - (1 vote) <\/div>\n <\/div>\n","protected":false},"excerpt":{"rendered":" Cambridge IGCSE Chemistry does not provide a dedicated formula sheet, so students are expected to memorise the key equations needed for calculations. The exam may provide useful information such as the Periodic Table, relative atomic masses, molar gas volume, or qualitative analysis tables depending on the paper, but the formulas themselves must come from memory. … Read more<\/a><\/p>\n","protected":false},"author":11,"featured_media":45397,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"content-type":"","rank_math_title":"","rank_math_description":"","footnotes":""},"categories":[166],"tags":[],"class_list":["post-45414","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-igcse"],"_links":{"self":[{"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/posts\/45414","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/users\/11"}],"replies":[{"embeddable":true,"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/comments?post=45414"}],"version-history":[{"count":3,"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/posts\/45414\/revisions"}],"predecessor-version":[{"id":45433,"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/posts\/45414\/revisions\/45433"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/media\/45397"}],"wp:attachment":[{"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/media?parent=45414"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/categories?post=45414"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/times.edu.vn\/en\/wp-json\/wp\/v2\/tags?post=45414"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}} |
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