Concentration (mol/dm³)Higher tier
AQA GCSE Chemistry calculation · part of the free Calculation Climb
Concentration measured in moles of solute per dm³ of solution: concentration = moles ÷ volume (dm³). It connects to g/dm³ through the relative formula mass: g/dm³ = mol/dm³ × Mr.
How to do it — step by step
- Convert the volume to dm³ (÷ 1000 if it's in cm³) — before anything else.
- concentration = moles ÷ volume (dm³). Rearrange as needed: moles = concentration × volume.
- To swap between the two concentration units: g/dm³ = mol/dm³ × Mr (and back again: ÷ Mr).
- Write what each number is ("moles = …", "conc = …") — unlabelled working loses partial credit.
Worked examples — from real AQA papers
Mass from a concentration. Calculate the mass of ethanedioic acid (H₂C₂O₄, Mr = 90) needed to make 250 cm³ of a 0.0480 mol/dm³ solution.
moles = 0.0480 × (250 ÷ 1000) = 0.012 mol → mass = 0.012 × 90 = 1.08 g
The examiner report on this exact question: far fewer students converted the volume to dm³ than usual, "so their calculated number of moles was 1000 times the correct value" — and "numbers were scattered around without helpful words such as 'moles =' or 'mass ='", which cost partial credit.AQA GCSE Chemistry Paper 1, Higher, 2021 — 2 marks
Unknown concentration from a titration. 25.00 cm³ of hydrochloric acid reacts with 23.50 cm³ of 0.100 mol/dm³ barium hydroxide. 2HCl + Ba(OH)₂ → BaCl₂ + 2H₂O.
moles Ba(OH)₂ = (23.50 ÷ 1000) × 0.100 = 0.00235 → moles HCl = 0.00235 × 2 = 0.00470 → conc = 0.00470 × 1000 ÷ 25.0 = 0.188 mol/dm³
The report's warning here is subtle: students who skip the cm³ → dm³ conversion often make the same error twice, "obtaining what looked like a correct answer" — with no working shown, that scores nothing. Show each step.AQA GCSE Chemistry Paper 1, Higher, 2022 — 4 marks
The full 6-marker: both units. 15.5 cm³ of 0.500 mol/dm³ sulfuric acid completely reacts with 25.0 cm³ of potassium hydroxide solution. 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O. Find the KOH concentration in mol/dm³ and g/dm³ (Ar: H = 1, O = 16, K = 39).
moles H₂SO₄ = 0.0155 × 0.500 = 0.00775 → moles KOH = 2 × 0.00775 = 0.0155 → conc = 0.0155 ÷ 0.025 = 0.62 mol/dm³ → Mr(KOH) = 56 → 0.62 × 56 = 34.7 g/dm³
About a third of students scored zero on this. The mol/dm³ answer was reached far more often than the final g/dm³ conversion — the × Mr step at the end is where the marks hide.AQA GCSE Chemistry Paper 1, Higher, 2019 — 6 marks
Where students lose marks — from the examiner reports
- Not converting cm³ to dm³. Named in the 2020, 2021 and 2022 reports — it makes every later number 1000× wrong. ÷ 1000 first, always.
- Applying the mole ratio to concentrations or volumes. The 2021 report: the ratio applies to moles — work in moles, then convert back to concentration at the end.
- Forgetting the × Mr step when g/dm³ is asked for (2019) — or dragging Mr in when it isn't needed (2021). Read what unit the answer wants.
- Unlabelled working. Multiple years: "numbers scattered around" and unexplained tables can't earn method marks, and an error that's invisible in the working kills every mark after it.
- Omitting the mole ratio entirely — flagged in the 2020 report on a 3 : 1 citric acid reaction.
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 Higher 2019 | 6 | Titration → concentration in mol/dm³ and g/dm³ → 0.62 / 34.7 |
| Chemistry Paper 1 Higher 2020 | 3 | Titration with a 3 : 1 ratio → 0.0798 mol/dm³ |
| Chemistry Paper 1 Higher 2021 | 2 | Mass to make 250 cm³ of a 0.0480 mol/dm³ solution → 1.08 g |
| Chemistry Paper 1 Higher 2021 | 3 | Titration with a 1 : 2 ratio → 0.0576 mol/dm³ |
| Chemistry Paper 1 Higher 2022 | 4 | Titration with a 2 : 1 ratio → 0.188 mol/dm³ |
The pattern is unmissable: this is a Higher-tier, Chemistry Paper 1 calculation, and it appeared every year from 2019 to 2022, almost always inside the titration question for 2–6 marks. Master the three-step chain on the titration page and this unit comes free with it.
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