Calculate the pH of 0.100 mol/dm3 ethanoic acid at 298 K. Ka = 1.74 x 10−5 mol/dm3.
[3 marks]
Total for this question: 3
All calculations
pH of a weak acid
Acids and bases
AQA 7405
ORIGINAL
[H+] = sqrt(Ka x [HA]); pH = -log10[H+]
A weak acid is only partly dissociated, so use Ka before taking the negative logarithm.
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
- Treating a weak acid as fully dissociated.
- Taking -log of Ka instead of the calculated hydrogen-ion concentration.
Exam tip: The square-root approximation is valid when dissociation is small; quote pH to two decimal places.
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