All calculations
Buffer pH
Acids and bases
AQA 7405
ORIGINAL
Ka = [H+][A-]/[HA], so [H+] = Ka[HA]/[A-]
Use the weak-acid equilibrium with the acid and conjugate-base concentrations after any neutralisation.
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
4 marks
A buffer contains 0.200 mol/dm3 CH3COOH and 0.150 mol/dm3 CH3COO-. Calculate its pH. Ka(CH3COOH) = 1.74 x 10−5 mol/dm3.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Inverting the acid-to-conjugate-base ratio.
- Using starting amounts without first allowing for added strong acid or alkali in a mixed problem.
Exam tip: If acid and salt share the same total volume, a mole ratio can replace their concentration ratio.
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.