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.
[4 marks]
Total for this question: 4
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.
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Work it, then mark it
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Q1
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.
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