Chemistry 0620 · syllabus 3
Cambridge IGCSE Chemistry 0620 — Stoichiometry topical past papers
Formulae and balanced equations, relative masses, and the mole: reacting masses, gas volumes, concentrations, titrations, empirical formulae and percentage yield.
- Questions
- 449
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 214
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What Stoichiometry covers
Stoichiometry is the arithmetic of chemistry, and it starts with formulae and equations: knowing the formulae of the substances in the course, deducing a formula from a model or diagram, and writing word equations and balanced symbol equations with state symbols, (s), (l), (g) and (aq). On the Extended route you also deduce the formula of an ionic compound from the charges on its ions, write ionic equations that leave out spectator ions, and work with the empirical formula, the simplest whole-number ratio of atoms or ions in a compound.
Relative atomic mass, Ar, compares the average mass of an element's atoms with one-twelfth of the mass of a carbon-12 atom, and adding up the Ar values in a formula gives the relative molecular mass, or for an ionic compound the relative formula mass, Mr. Core candidates use these to calculate reacting masses in simple proportion, without moles, and know that the concentration of a solution can be given in g/dm³ or mol/dm³.
The mole is Extended content. One mole of a substance contains 6.02 × 10²³ particles, the Avogadro constant, and has a mass in grams equal to its Ar or Mr. With amount = mass ÷ molar mass, the molar gas volume of 24 dm³ at room temperature and pressure, and concentration = amount ÷ volume in dm³, you calculate reacting masses, limiting reactants, gas volumes, concentrations from titration results, empirical and molecular formulae, and percentage yield, composition by mass and purity. These calculations run through Papers 2 and 4, often as one part of a question about another topic.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 3 Stoichiometry
- 3.1 Formulae
- 3.2 Relative masses of atoms and molecules
- 3.3 The mole and the Avogadro constant
What you should be able to do
- Write balanced symbol equations with state symbols, and ionic equations on the Extended route
- Calculate Mr from a formula, and reacting masses in simple proportion
- Convert between mass, amount in moles, number of particles and, for gases at r.t.p., volume (Extended)
- Use the mole ratio in a balanced equation to find the mass or volume of a product and to identify the limiting reactant (Extended)
- Calculate concentrations in mol/dm³ and g/dm³, converting cm³ to dm³, including from titration results (Extended)
- Find empirical and molecular formulae, and calculate percentage yield, percentage composition by mass and percentage purity (Extended)
Key ideas
- Amount of substance (mol) = mass (g) ÷ molar mass (g/mol)
- One mole contains 6.02 × 10²³ particles; one mole of any gas occupies 24 dm³ at r.t.p.
- Concentration (mol/dm³) = amount (mol) ÷ volume (dm³), and 1 dm³ = 1000 cm³
- Percentage yield = actual yield ÷ theoretical yield × 100; percentage purity = mass of pure substance ÷ mass of sample × 100
- The molecular formula is a whole-number multiple of the empirical formula; the multiple is Mr ÷ the mass of the empirical formula
How Stoichiometry has been examined
From 2022 to 2026, Stoichiometry was on Paper 4 · Theory (Extended) in 13 of 14 exam series, worth about 24% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 56 | 4% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 74 | 6% | 14 of 14 |
| Paper 3 · Theory (Core) | 35 | 15% | 11 of 14 |
| Paper 4 · Theory (Extended) | 44 | 24% | 13 of 14 |
| Paper 6 · Alternative to Practical | 5 | 4% | 3 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1.
Filed in Momentum under: Reacting Masses and Chemical Equations, Using Moles.
A worked example
4.80 g of magnesium is added to 100 cm³ of 2.00 mol/dm³ hydrochloric acid: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Find the limiting reactant, the volume of hydrogen formed at r.t.p., and the mass of the other reactant left over.
- Moles of magnesium = 4.80 ÷ 24 = 0.200 mol
- Moles of HCl = 2.00 × (100 ÷ 1000) = 0.200 mol
- 0.200 mol of magnesium would need 2 × 0.200 = 0.400 mol of HCl, but only 0.200 mol is present, so the acid is the limiting reactant
- Moles of H₂ = 0.200 ÷ 2 = 0.100 mol, so the volume = 0.100 × 24 = 2.40 dm³
- Magnesium that reacts = 0.100 mol = 0.100 × 24 = 2.40 g, so 4.80 − 2.40 = 2.40 g of magnesium is left over
Answer: Hydrochloric acid is the limiting reactant; 2.40 dm³ of hydrogen at r.t.p.; 2.40 g of magnesium is left unreacted
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Leaving a volume in cm³ in a concentration or gas calculation — 25.0 cm³ is 0.0250 dm³, and missing the factor of 1000 makes the answer a thousand times too large or too small
- Ignoring the mole ratio — in Mg + 2HCl → MgCl₂ + H₂ one mole of magnesium needs two moles of acid, so copying moles straight across picks the wrong limiting reactant
- Using the Ar of oxygen or chlorine for the gas — O₂ has Mr 32 and Cl₂ has Mr 71, because these elements exist as diatomic molecules
- Rounding a ratio such as 1 : 1.5 to 1 : 2 when finding an empirical formula — multiply through instead, to 2 : 3; only values very close to whole numbers can be rounded
- Rounding part-way through a calculation — carry the full values and round only the final answer, or the last figure drifts away from the mark scheme's
Exam technique
- Set out every mole calculation in the same three steps: moles of the substance you know, the mole ratio from the equation, then the mass, volume or concentration asked for
- Write units on every line (g, mol, dm³, mol/dm³); checking them catches most cm³ and dm³ slips
- For a limiting reactant, work out the moles of each reactant and divide by its number in the balanced equation; the smaller result shows the limiting reactant
- Give the final answer to the number of significant figures in the data, usually three, keeping more figures until the last line
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.