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Chemistry Paper 1 / Quantitative chemistry
Lesson 3.11 · AQA Combined Science

Higher Tier: equations from masses

Higher Tier only. This is an optional extension after the core lessons. Go to exam practice.

Work backwards from measured masses

Higher Tier only. Given the masses of substances that react or form, first convert each mass to moles. Then divide each amount by the smallest to find a simple ratio. Use that ratio as the equation coefficients.

Worked example

24 g magnesium reacts with 16 g oxygen to make 40 g magnesium oxide. Molar masses: Mg 24, O₂ 32, MgO 40 g/mol.
Moles: 24/24 = 1; 16/32 = 0.5; 40/40 = 1.
Divide every amount by 0.5: 2:1:2.
Equation: 2Mg + O₂ → 2MgO.

Convert fractional ratios to whole numbers

If dividing gives 1:1.5:1, multiply every value by 2 to give 2:3:2. Do not round 1.5 to 2.
Example: 2Fe + 3Cl₂ → 2FeCl₃. The formula Cl₂ matters: use molar mass 71, not 35.5.

Check the result

Check every element count after you write the equation. Small measurement differences may make a ratio close to a whole number, such as 1.99; rounding to 2 can be justified by experimental uncertainty. A ratio of 1.50 needs scaling, not careless rounding. Use reacting masses, not the mass of an excess reactant left over.

Remember

Masses → moles → divide by the smallest → whole-number ratio → check atoms.