Formulae, Equations and the Mole

This guide covers topic 3, Stoichiometry for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series.

Read this before you revise. This is the topic where the two qualifications diverge most. In 0620, almost all of the mole content is Extended — the only Core outcome in subtopic 3.3 is that concentration can be measured in g/dm³ or mol/dm³. In 5070, the entire mole content is required.

So:

• IGCSE Core candidates need formulae, relative masses and simple reacting masses — but are not required to do mole calculations, molar gas volumes, empirical formulae from data, titration calculations or percentage yield.
• IGCSE Extended candidates need all of it.
• O Level 5070 candidates need all of it. There is no lower tier.

Do not use a generic “IGCSE and O Level” mole guide that ignores this. It will either overload a Core candidate or under-prepare an O Level one.

3.1 Formulae

CORE (0620) · REQUIRED (5070) — state the formulae of the elements and compounds named in the subject content; define molecular formula as the number and type of different atoms in one molecule; deduce the formula of a simple compound from the relative numbers of atoms present; and construct word equations and symbol equations.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — define empirical formula as the simplest whole number ratio of the different atoms or ions in a compound; deduce the formula of an ionic compound from the charges on the ions; construct symbol equations with state symbols including ionic equations; and deduce symbol equations with state symbols from given information.

Note that state symbols appear in both tiers of 0620: Core outcome 4 covers word and symbol equations including state symbols, and the Supplement extends this to ionic equations. Ionic equations are required for O Level 5070 as well.

Deducing a formula from a model or diagram

A question may show a ball-and-stick model, a space-filling model or a diagram of a lattice instead of giving the formula. Count the atoms (or ions) of each element and write them as the formula.

• A molecule (Core, 0620 3.1.3): a model shows one nitrogen atom bonded to three hydrogen atoms. The formula is NH₃. A model with two carbon atoms and six hydrogen atoms is C₂H₆. Count every atom in the molecule; do not simplify a molecular formula.
• An ionic lattice (Extended, 0620 3.1.6): a diagram of part of a lattice shows the ions in the ratio one Mg²⁺ to two Cl⁻. The formula is MgCl₂: an ionic formula shows the simplest ratio of ions, and a lattice has no fixed number of ions.

5070 has no tiers, so O Level candidates need both.

Deducing ionic formulae from charges

The compound must be electrically neutral, so total positive charge equals total negative charge.

Magnesium forms Mg²⁺ and chloride is Cl⁻. One Mg²⁺ needs two Cl⁻ to balance, giving MgCl₂. Aluminium Al³⁺ with oxide O²⁻ requires the lowest common multiple of 3 and 2, which is 6: two Al³⁺ and three O²⁻, giving Al₂O₃.

Balancing equations

Balance atoms of each element on both sides by changing coefficients only. Never change a subscript inside a formula — that changes the substance.

CH₄ + 2O₂ → CO₂ + 2H₂O

State symbols are (s) solid, (l) liquid, (g) gas, (aq) aqueous.

3.2 Relative masses of atoms and molecules

CORE (0620) · REQUIRED (5070) — describe relative atomic mass, Ar, as the average mass of the isotopes of an element compared to 1/12 of the mass of a carbon-12 atom; define relative molecular mass, Mr, as the sum of the relative atomic masses. Relative formula mass is used for ionic compounds.

0620 also lists calculating reacting masses in simple proportions as Core here — so a Core candidate does meet reacting-mass arithmetic, just without the mole.

For CaCO₃: Ar(Ca) = 40, Ar(C) = 12, Ar(O) = 16, so Mr = 40 + 12 + (3 × 16) = 100.

3.3 The mole and the Avogadro constant

CORE (0620) — one outcome only: state that concentration can be measured in g/dm³ or mol/dm³.

EXTENDED / SUPPLEMENT (0620) · ALL REQUIRED FOR 5070 — everything that follows in this section.

The mole

The mole (mol) is the unit of amount of substance. One mole contains 6.02 × 10²³ particles — the Avogadro constant.

amount (mol) = mass (g) ÷ molar mass (g/mol)

How many moles in 50 g of calcium carbonate? Mr = 100, so 50 ÷ 100 = 0.5 mol.

Molar gas volume

The molar gas volume is taken as 24 dm³ at room temperature and pressure (r.t.p.).

volume of gas at r.t.p. (dm³) = amount (mol) × 24

2 mol of any gas occupies 48 dm³ at r.t.p.

Reacting masses and limiting reactants

Work in three steps: convert the known mass to moles, use the balanced equation’s ratio to find moles of the unknown, then convert back to mass or volume.

For CaCO₃ → CaO + CO₂, decomposing 50 g of CaCO₃ gives 0.5 mol CaCO₃. The ratio is 1:1, so 0.5 mol of CO₂ forms, occupying 0.5 × 24 = 12 dm³ at r.t.p.

Where two reactants are given, identify the limiting reactant — the one that runs out first — and base the calculation on it.

Concentration and titration

concentration (mol/dm³) = amount (mol) ÷ volume (dm³)

Remember to convert cm³ to dm³ by dividing by 1000. Titration calculations use experimental data to find an unknown concentration.

Empirical and molecular formulae

From percentage or mass data: divide each mass by that element’s Ar, then divide all results by the smallest to get the simplest whole-number ratio.

The molecular formula is a whole-number multiple of the empirical formula, found by comparing the Mr of the compound with the empirical formula mass.

Percentage yield, composition and purity

Percentage yield compares the actual yield with the theoretical maximum. Percentage composition by mass and percentage purity are also required at this level. Each is a proportion expressed as a percentage — set out the numerator and denominator explicitly before dividing.

Common mistakes

• Changing subscripts to balance an equation. Only coefficients may change.
• Forgetting to convert cm³ to dm³ in concentration calculations.
• Using 24 dm³ for a liquid or solid. The molar gas volume applies to gases at r.t.p.
• Mixing up empirical and molecular formula. Empirical is the simplest ratio.
• IGCSE Core candidates attempting full mole calculations they are not required to do — or O Level candidates skipping them because a guide called them “Extended”.

Quick revision checklist

All candidates: formulae of named substances · molecular formula defined · deducing simple formulae · word and symbol equations, including state symbols · Ar and Mr · simple reacting masses

0620 Extended and all 5070 candidates, additionally: empirical formula defined · ionic formulae from charges · the mole and Avogadro constant · molar gas volume at r.t.p. · stoichiometric reacting masses and limiting reactants · titration calculations · empirical and molecular formulae from data · percentage yield, composition and purity

Ionic equations: 0620 Extended and all 5070 candidates

Related resources

• Atomic Structure — Ar and isotopes first
• Ionic, Covalent and Metallic Bonding — ionic charges
• Redox Reactions — oxidation numbers build on the formulae covered here
• Rates of Reaction and Reversible Reactions — balancing the Haber and Contact process equations
• Atoms, Molecules and Stoichiometry at AS — the AS Level continuation of this topic (9701)
• Quantitative Chemistry (OxfordAQA IGCSE) — the equivalent topic from a different awarding body
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.