Calculate the pH of 0.050 mol/dm3 hydrochloric acid. Give your answer to 2 decimal places.
[3 marks]
Total for this question: 3
All calculations
pH of a strong acid
Physical chemistry
AQA 7405
ORIGINAL
pH = -log10[H+] (strong monoprotic acid: [H+] = concentration of acid)
For a strong monoprotic acid [H+] equals the acid concentration — then it's just -log.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
Do the calculation on paper first — then mark it.
Where the marks get lost
- Forgetting the minus sign in pH = -log10[H+].
- Assuming [H+] equals the concentration for a WEAK acid — that only holds for strong acids.
- Giving pH to the wrong precision — quote pH to 2 decimal places.
Exam tip: Only a STRONG monoprotic acid has [H+] equal to its concentration. A fully dissociating diprotic acid doubles [H+] (H2SO4 is usually treated this way as an approximation — its second ionisation isn't quite complete); a weak acid needs Ka.
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