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AQA and OCR A · Year 13

Acid–base calculation questions

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Acid–base calculation trainer

Guided set

Question1 / 5 Marks0 / 0 Clues0
AQA-style practice [5 marks]

Acids 0.1.0 · site 1.20.0

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  1. Work out what reacts completely before using an equilibrium expression.
  2. Decide whether the final mixture contains excess strong acid, excess strong base, a weak acid or a buffer.
  3. Use Ka = [H+][A−]/[HA] or Kw = [H+][OH−] as appropriate.
  4. Convert the final hydrogen-ion concentration into pH.

Buffer calculations

For an acidic buffer, rearrange the acid dissociation expression to give [H+] = Ka[HA]/[A−]. When both components are in the same final volume, their mole ratio can be used because that volume cancels.

Common slips

  • Using concentrations before completing the neutralisation calculation.
  • Forgetting the total volume after two solutions are mixed.
  • Changing only one buffer component when acid or base is added.
  • Using 14 instead of the supplied value of Kw at another temperature.
  • Treating calcium hydroxide as though it supplied one hydroxide ion per formula unit.

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Kc and equilibrium

Equilibrium expressions

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