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AS · Practice questions · Brønsted–Lowry acids and bases

Follow the H+.

Six original Cambridge-style questions on Brønsted–Lowry theory: acids, bases, conjugate pairs, and strong and weak acids.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Acid gives H+; base takes it.

01
Recall
[2 marks]

Define a Brønsted–Lowry acid and a Brønsted–Lowry base.

  • Acid: a proton (H+) donor. ✓
  • Base: a proton (H+) acceptor. ✓
02
Analysis
[2 marks]

Identify the two conjugate acid-base pairs in CH3COOH + H2O ⇌ CH3COO− + H3O+.

  • CH3COOH (acid) and CH3COO− (conjugate base). ✓
  • H2O (base) and H3O+ (conjugate acid). ✓
03
Explain
[2 marks]

Use equations to show that water can act as both an acid and a base.

  • As a base: HCl + H2O → H3O+ + Cl− (water accepts H+). ✓
  • As an acid: NH3 + H2O ⇌ NH4+ + OH− (water donates H+). ✓
04
Explain
[2 marks]

Explain the difference between a strong acid and a weak acid, using HCl and CH3COOH as examples with equations.

  • HCl is fully dissociated: HCl → H+ + Cl−. ✓
  • CH3COOH is only partially dissociated: CH3COOH ⇌ CH3COO− + H+, with the equilibrium well to the left. ✓
05
Explain
[3 marks]

Describe two experiments that would distinguish 0.10 mol dm−3 hydrochloric acid from 0.10 mol dm−3 ethanoic acid, giving the result for each acid.

  • Measure pH: HCl about 1, ethanoic acid about 3 (higher [H+] in the strong acid). ✓
  • Add magnesium ribbon: faster effervescence with HCl. ✓
  • Or measure conductivity: higher for HCl because it has more ions. ✓
06
Analysis
[2 marks]

Give the conjugate base of (a) HSO4− and (b) NH4+, and the conjugate acid of (c) CO32−.

  • (a) SO42−; (b) NH3. ✓
  • (c) HCO3−. ✓

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