Percentage by mass
AQA GCSE Chemistry calculation · part of the free Calculation Climb
The percentage of a compound's mass that comes from one element — and, on AQA papers, its twin: the percentage of a mixture (an alloy, an ore, a fuel) made of one part. Same maths, and between them they've appeared every single year from 2019 to 2025 — this is the most-examined calculation in GCSE Chemistry.
How to do it — step by step
- Element in a compound: total mass of that element = Ar × how many atoms of it; work out the Mr of the whole compound; % = (element mass ÷ Mr) × 100.
- Part of a mixture or alloy: % = (part ÷ whole) × 100 — and in reverse, mass of part = (% ÷ 100) × total.
- Get every mass into the same unit before dividing (mg vs g, kg vs g).
- If the question asks for significant figures or standard form, that final step is a mark of its own.
Worked examples — from real AQA papers
Element in a compound (Mr given). Calculate the percentage by mass of copper in copper sulfate. Ar Cu = 63.5; Mr CuSO₄ = 159.5. Give your answer to 3 significant figures.
% = (63.5 ÷ 159.5) × 100 = 39.81… = 39.8%
One atom of copper, so it's a single division — the marks are for the substitution, the unrounded value, and the 3-s.f. rounding.AQA GCSE Chemistry Paper 1, Foundation, 2024 — 3 marks
Element in a compound (you build the Mr). Calculate the percentage by mass of chlorine in C₆H₁₀Cl₂. Ar: H = 1, C = 12, Cl = 35.5.
Mr = (6 × 12) + (10 × 1) + (2 × 35.5) = 153 → % = (71 ÷ 153) × 100 = 46.4%
Over half of students scored all three marks — but the examiner report notes some used 35 for chlorine even though 35.5 was given in the question. Use the numbers on the paper, not the ones you half-remember.AQA GCSE Chemistry Paper 2, Higher, 2021 — 3 marks
Alloy, in reverse. A 5.25 g sample of a copper–zinc alloy is 13.5% zinc. Calculate the mass of copper, to 3 significant figures.
% copper = 100 − 13.5 = 86.5% → mass = (86.5 ÷ 100) × 5.25 = 4.54125 = 4.54 g
The percentage you need isn't the one printed — subtract from 100 first. The report notes students who showed working "often gained credit even after making errors".AQA GCSE Combined Science Chemistry Paper 1, Foundation, 2024 — 4 marks
Where students lose marks — from the examiner reports
- Not counting every atom. On the 2023 solder question (6 lead atoms per 24 tin), "a response of 25% was fairly common, as a result of assuming the total number of atoms was 24" instead of 30. The denominator is the whole, always.
- Missing the unit conversion. The 2023 water-vapour question (answer in grams, data in kg) had under 20% full marks; the 2022 fuels question (kg → g) about 25%. Convert — and show the conversion, it's usually a mark.
- Using the wrong Ar. 35 instead of the given 35.5 for chlorine (2021). The values are printed on the paper for a reason.
- Not deriving the percentage first. The 2023 titanium alloy gave the other metals' percentages — titanium was 100 − 3.0 − 2.5 = 94.5%. Only about a quarter completed it.
- Dropping the final rounding mark. 3 s.f. (2024), 2 s.f. (2023), standard form (2020, 2022) — each is a dedicated mark on the scheme.
How often it comes up on AQA papers
From Science Street's database of real AQA questions:
| Paper | Marks | What you had to do |
|---|---|---|
| Chemistry Paper 1 Foundation 2019 | 2 | 38% of a 40 kg rock sample → 15.2 kg |
| Chemistry Paper 2 Foundation 2019 | 3 | 700 mg in a 1.20 g tablet (convert!) → 58.3% |
| Chemistry Paper 1 Foundation 2020 | 3 | 25% of 300 000 kg, in standard form → 7.5 × 10⁴ kg |
| Chemistry Paper 2 Foundation 2021 | 2 | 5.94 kg of 6.00 kg alloy → 99% |
| Chemistry Paper 2 Higher 2021 | 3 | Cl in C₆H₁₀Cl₂ → 46.4% |
| Chemistry Paper 2 Foundation 2022 | 3 | 5% of 4.4 kg of E5 fuel, in grams → 220 g |
| Combined Science Chemistry Paper 2 Higher 2022 | 4 | Phytomining two-step, standard form → 1.65 × 10⁵ g |
| Combined Science Chemistry Paper 1 Foundation 2023 | 4 | O in Li₂SO₄, to 2 s.f. → 58% |
| Chemistry Paper 1 Foundation 2023 | 3 | Lead atoms in solder (6 of 30) → 20% |
| Chemistry Paper 2 Higher 2023 | 3 | Cu in CuFeS₂ (build the Mr) → 34.6% |
| Chemistry Paper 2 Foundation 2023 | 3 + 3 | Titanium alloy (derive 94.5%) → 4.725 kg; water vapour, in grams → 875 g |
| Chemistry Paper 1 Foundation 2024 | 3 | Cu in CuSO₄, 3 s.f. → 39.8% |
| Combined Science Chemistry Paper 1 Foundation 2024 | 4 | Alloy in reverse, 3 s.f. → 4.54 g |
| Combined Science Chemistry Paper 2 Higher 2024 | 2 | 77.8% of 3.50 g of dissolved solids → 2.72 g |
| Chemistry Paper 2 Foundation 2025 | 2 | 75% of a 4 g gold ring → 3 g |
The pattern: it appears at both tiers, on both papers, every year — usually 2–4 marks. Foundation leans on mixtures and alloys with the percentage given (or one subtraction away); Higher makes you build the Mr yourself. The extras that separate the marks are always the same three: a unit conversion, a subtraction from 100, or a rounding instruction.
Now practise it — free
Science Street's Calculation Climb walks you from a worked example, through a guided step-by-step version, to questions you do yourself — with fresh examples every time and a built-in calculator and periodic table. No sign-up needed.
Practise Percentage by mass on Science Street →