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.
Verified against AQA 7405 (2026 spec)
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
3 marks
Calculate the pH of 0.100 mol/dm3 ethanoic acid at 298 K. Ka = 1.74 x 10−5 mol/dm3.
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.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.