Percentage yield
AQA GCSE Chemistry calculation · part of the free Calculation Climb
How much product you actually got, compared with the maximum you could in theory have made: % yield = (mass actually made ÷ maximum theoretical mass) × 100. Both masses are masses of the product — the mass of reactant you started with never goes in the fraction.
How to do it — step by step
- Get both masses into the same unit (1 kg = 1000 g).
- Identify the two product masses: what was actually made and the maximum theoretical mass.
- % yield = (actual ÷ theoretical) × 100. Actual goes on top — a yield over 100% means the fraction is upside down.
- If the question asks for significant figures, round only at the end.
Worked examples — from real AQA papers
The standard version — watch the units. 500 g of titanium dioxide should in theory produce 1.2 kg of titanium chloride, but only 900 g is actually made.
1.2 kg = 1200 g → % yield = (900 ÷ 1200) × 100 = 75%
The examiner report said this "discriminated well" with only 32% getting both marks: many students put the 500 g of reactant into the fraction, inverted the expression, or tried subtracting. The 500 g is context, not data — only the two product masses matter.AQA GCSE Chemistry Paper 1, Foundation, 2018 — 2 marks
In reverse — finding the actual mass. The maximum theoretical mass of a salt is 12.5 g and the percentage yield is 92.8%. Find the mass actually produced.
mass = (92.8 ÷ 100) × 12.5 = 11.6 g
Same equation, rearranged. The examiner report said rearranging "proved difficult for some" and that students kept trying to work the 50 cm³ of acid from earlier in the question into the calculation — AQA regularly includes numbers you don't need.AQA GCSE Chemistry Paper 1, Foundation, 2024 — 3 marks
Higher tier: theoretical mass via moles first. 6.35 g of iodine reacts with zinc (Zn + I₂ → ZnI₂) and 6.29 g of zinc iodide is produced. Mr: I₂ = 254, ZnI₂ = 319. Find the % yield to 3 significant figures.
moles I₂ = 6.35 ÷ 254 = 0.025 → theoretical ZnI₂ = 0.025 × 319 = 7.975 g → % yield = (6.29 ÷ 7.975) × 100 = 78.87… = 78.9%
Nearly half of students got full marks, but the report warns the working "has to make sense chemically" — plugging numbers together in a way that happens to reach 78.9 was capped at the sig-figs mark.AQA GCSE Chemistry Paper 1, Higher, 2025 — 5 marks
Where students lose marks — from the examiner reports
- Putting the reactant mass in the fraction. The single most-reported error (2018 report). Percentage yield compares product with product.
- Inverting the fraction or subtracting. Also from the 2018 report. Actual ÷ theoretical, times 100 — nothing else.
- Mixing kg and g. The 2018 question deliberately gave one mass in kg and one in g; converting was worth a mark on its own.
- Ignoring the significant-figures instruction. On the 2019 paper (28.4 ÷ 31.8 × 100 = 89.3% to 3 s.f.), a common error was writing "89.3 with a dot over the 3" — the recurring-decimal notation isn't what "3 significant figures" means.
- Failing the rearrangement. When the yield is given and a mass is asked for (2024), many students couldn't rearrange — practise both directions.
How often it comes up on AQA papers
From Science Street's database of real AQA questions, percentage yield has appeared on these papers since 2018:
| Paper | Marks | What you had to do |
|---|---|---|
| Chemistry Paper 1 Foundation 2018 | 2 | Convert kg → g, then (900 ÷ 1200) × 100 = 75% |
| Chemistry Paper 1 Foundation 2019 | 3 | (28.4 ÷ 31.8) × 100 = 89.3% to 3 significant figures |
| Chemistry Paper 2 Foundation 2021 | 2 | (380 ÷ 400) × 100 = 95% for ethanol production |
| Chemistry Paper 1 Foundation 2024 | 3 | Reverse: 92.8% of a 12.5 g theoretical maximum = 11.6 g |
| Chemistry Paper 1 Foundation 2025 | 2 | (13.6 ÷ 16.0) × 100 = 85% after separating a mixture |
| Chemistry Paper 1 Higher 2025 | 5 | Moles → theoretical mass → % yield, to 3 s.f. |
The pattern: at Foundation the two masses are given (sometimes in different units, sometimes with the equation printed) for 2–3 marks. At Higher it becomes a multi-step calculation — find the theoretical mass through moles first — worth up to 5 marks.
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