Concentration (g/dm³)
AQA GCSE Chemistry calculation · part of the free Calculation Climb
How much solute is dissolved per dm³ of solution: concentration = mass (g) ÷ volume (dm³). Remember 1000 cm³ = 1 dm³ — nearly every real exam question hides the volume conversion inside it.
How to do it — step by step
- Convert the volume to dm³ (÷ 1000 if it's in cm³).
- concentration = mass (g) ÷ volume (dm³).
- Rearrange when the question flips: mass = concentration × volume (dm³).
- If the question gives repeat readings, find the mean first, then do the concentration step.
Worked examples — from real AQA papers
Finding the concentration. 25 cm³ of copper sulfate solution contains 6.75 g of copper sulfate. Find the concentration in g/dm³.
25 ÷ 1000 = 0.025 dm³ → concentration = 6.75 ÷ 0.025 = 270 g/dm³
Over half of students got this fully right — but the examiner report says many "could not convert their volume to dm³ even given the conversion factor" in the question. The conversion is the first mark; show it.AQA GCSE Chemistry Paper 1, Foundation, 2024 — 3 marks
In reverse — finding the mass. A potash alum solution has concentration 258 g/dm³. What mass is needed to make 800 cm³, to 3 significant figures?
800 ÷ 1000 = 0.8 dm³ → mass = 0.8 × 258 = 206.4 = 206 g
Same equation, rearranged — and the final mark was for rounding 206.4 to 3 significant figures.AQA GCSE Chemistry Paper 2, Foundation, 2021 — 4 marks
Higher: concentration at the end of a moles chain. 0.72 g of magnesium is added to 100 cm³ of excess hydrochloric acid. Find the concentration of the MgCl₂ solution produced (Ar Mg = 24, Mr MgCl₂ = 95).
moles Mg = 0.72 ÷ 24 = 0.03 → mass MgCl₂ = 0.03 × 95 = 2.85 g → concentration = 2.85 ÷ 0.100 = 28.5 g/dm³
The report calls this "a good discriminator": a third got the first moles mark, about 20% reached 2.85 g, and only a small percentage finished — most stumbled at converting 100 cm³ for the final division.AQA GCSE Combined Science Chemistry Paper 1, Higher, 2023 — 6 marks
Where students lose marks — from the examiner reports
- The cm³ → dm³ conversion. The recurring theme across 2023, 2024 and Higher papers — including students who fail it when the factor is printed in the question. ÷ 1000, first, every time.
- Dividing by the wrong power of ten. The 2024 Combined report lists the exact slips: multiplying by 1000 instead of dividing, and dividing by 100 instead of 1000.
- Multiplying when you should divide (and vice versa). Going volume × concentration when the question wants mass ÷ volume — decide first whether the answer should be a mass or a concentration.
- Skipping the mean. Since 2024, AQA has liked giving four trial results first: the first marks are for a correct mean, and only then the concentration step.
- Sig figs at the end. When the question says 3 significant figures (2021), the rounding is its own mark.
How often it comes up on AQA papers
From Science Street's database of real AQA questions — this is one of the most frequently examined calculations in GCSE Chemistry:
| Paper | Marks | What you had to do |
|---|---|---|
| Chemistry Paper 2 Foundation 2020 | 3 | Scale 20 g in 400 cm³ up to 1.0 dm³ → 50 g |
| Chemistry Paper 2 Foundation 2021 | 4 | Reverse: mass for 800 cm³ at 258 g/dm³, 3 s.f. → 206 g |
| Combined Science Chemistry Paper 1 Foundation 2022 | 3 | 0.86 g in 25 cm³ → 34.4 g per dm³ |
| Chemistry Paper 2 Foundation 2022 | 2 | Scale 1.50 g in 50 cm³ up to 1000 cm³ → 30 g |
| Combined Science Chemistry Paper 1 Foundation 2023 | 3 | 0.30 g in 25 cm³ → 12 g/dm³ |
| Combined Science Chemistry Paper 1 Higher 2023 | 6 | Moles → mass → concentration → 28.5 g/dm³ |
| Chemistry Paper 1 Foundation 2024 | 3 | 6.75 g in 25 cm³ → 270 g/dm³ |
| Combined Science Chemistry Paper 1 Foundation 2024 | 3 | Reverse: mass in 30 cm³ at 180 g/dm³ → 5.4 g |
| Chemistry Paper 2, both tiers, 2024 | 4 | Mean of 4 trials, then × 39.3% for sodium ions → 13.8 g/dm³ |
| Chemistry Paper 2 Foundation 2025 | 5 | Mean of 4 masses, convert 10 cm³, divide → 34.75 g/dm³ |
The pattern: it has appeared every year from 2020 to 2025, on both papers and at both tiers, for 2–6 marks. Foundation gets a direct calculation or a reversal; Higher chains it onto moles; and recent papers wrap it in practical data where you take a mean first. If you learn one chemistry calculation cold, make it this one.
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