Reacting massesHigher tier
AQA GCSE Chemistry calculation · part of the free Calculation Climb
Use the balanced equation's ratio to find how much product forms (or reactant is needed). At Higher this is the full moles chain — mass → moles → ratio → mass. At Foundation, AQA asks the same idea as simple proportion: scale a given mass ratio up or down.
How to do it — step by step
- Work out the Mr of the substance whose mass you know (and convert kg → g first: × 1000).
- moles of what you know = mass ÷ Mr.
- Use the equation ratio to get moles of the other substance — check which way the ratio goes.
- mass = moles × Mr of the other substance.
- Foundation shortcut: when the question gives a mass ratio or a "this much makes that much" pair, scale it: (target ÷ known) × given mass.
Worked examples — from real AQA papers
Foundation: scaling a ratio. Propene and water react in the mass ratio propene : water = 7 : 3. Calculate the mass of propene that reacts with 21 g of water.
mass = (7 ÷ 3) × 21 = 49 g
No moles needed — recognise it's a proportion, keep the ratio the right way up, and multiply.AQA GCSE Chemistry Paper 2, Foundation, 2019 — 2 marks
Higher: the full chain. N₂ + 3H₂ → 2NH₃. Calculate the mass of hydrogen needed to produce 25 g of ammonia. Ar: H = 1, N = 14.
Mr(NH₃) = 17 → moles NH₃ = 25 ÷ 17 = 1.47 → moles H₂ = 1.47 × 3/2 = 2.205 → mass = 2.205 × 2 = 4.41 g
All four marks map to the four steps: Mr, moles, ratio, mass. Write each line labelled and the marks follow even if one number slips.AQA GCSE Combined Science Chemistry Paper 1, Higher, 2024 — 4 marks
Higher with a unit conversion. 2Mg + SiO₂ → Si + 2MgO. Calculate the minimum mass in grams of magnesium needed to completely reduce 1.2 kg of silicon dioxide. Ar: O = 16, Mg = 24, Si = 28.
Mr(SiO₂) = 60 → 1.2 kg = 1200 g → moles SiO₂ = 1200 ÷ 60 = 20 → moles Mg = 20 × 2 = 40 → mass = 40 × 24 = 960 g
Five marks, and one of them is just the kg → g conversion.AQA GCSE Chemistry Paper 1, Higher, 2022 — 5 marks
Where students lose marks — from the examiner reports
- Getting the Mr wrong at step one. On the 2019 electrolysis 4-marker, Mr(Al₂O₃) = 156 was "a common error" (it's 102) — but students who showed working still scored the later marks with their wrong value carried forward. Showing each step protects you.
- Ignoring what the question tells you to use. The 2018 report: students were instructed to use their previous answer, but "used a different point on the graph, which almost always led to an incorrect answer".
- Using the wrong given mass. On the 2023 reversible-reactions question, "a large proportion" used 4.68 g instead of 2.99 g, not realising the calculation was about the reverse reaction. Match the mass to the direction.
- Unclear working. The 2019 equation-detective question (which of two equations fits the data — about half scored zero): "students scored best when their working was set out clearly". These are multi-step calculations; a jumble of numbers can't earn method marks.
- Skipping the kg → g conversion (2022) — worth a mark by itself.
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 2018 | 2 | Scale up from a graph reading → 80 g |
| Chemistry Paper 1 Higher 2019 | 4 | 2Al₂O₃ → 4Al + 3O₂: oxygen from 2000 kg → 941 kg |
| Chemistry Paper 1 Higher 2019 | 3 | Decide which of two equations fits the measured masses |
| Chemistry Paper 2 Foundation 2019 | 2 | Ratio 7 : 3 scaled to 21 g → 49 g |
| Chemistry Paper 1 Higher 2021 | 3 | Moles from a graph, then × 6.02 × 10²³ → 2.28 × 10²¹ atoms |
| Chemistry Paper 1 Higher 2022 | 5 | 2Mg + SiO₂, kg → g → 960 g |
| Chemistry Paper 2 Higher 2023 | 3 + 2 | Hydrated copper sulfate: water by scaling → 4.23 g; energy by scaling → 7.37 kJ |
| Combined Science Chemistry Paper 1 Higher 2024 | 4 | N₂ + 3H₂ → 2NH₃: hydrogen for 25 g → 4.41 g |
| Chemistry Paper 2 Foundation 2024 | 2 | 7000 kg CO₂ per 3000 kg coal → 2.33 kg per kg |
| Chemistry Paper 2 Foundation 2025 | 2 | Energy scaled by mass: (4 ÷ 20) × 23 → 4.6 kJ |
The pattern: the full moles chain is a Higher-tier Paper 1 staple worth 3–5 marks, while Foundation gets proportion versions on Paper 2 for 2 marks. AQA also likes dressing the same scaling skill in energy clothing (kJ instead of g — 2023, 2025). Learn the four-step chain and both costumes fall to it.
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.
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