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IGCSE · Practice questions · Acids as proton donors, strong and weak acids

Strong is not concentrated.

Six original Cambridge-style questions on proton transfer and strong and weak acids. Extended content.

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

Donate, accept, dissociate.

01
Recall
[2 marks]

(Extended) Define an acid and a base in terms of proton transfer.

  • An acid is a proton (H+) donor. ✓
  • A base is a proton (H+) acceptor. ✓
02
Recall
[2 marks]

(Extended) Write equations to show the dissociation of (a) hydrochloric acid and (b) ethanoic acid in water.

  • (a) HCl(aq) → H+(aq) + Cl−(aq) ✓
  • (b) CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq) ✓
03
Explain
[3 marks]

(Extended) Explain why 0.1 mol/dm3 ethanoic acid has a higher pH than 0.1 mol/dm3 hydrochloric acid.

  • Ethanoic acid is a weak acid; it is only partially dissociated. ✓
  • Hydrochloric acid is a strong acid and is completely dissociated. ✓
  • So ethanoic acid has a lower concentration of H+ ions, giving a higher pH. ✓
04
Analysis
[3 marks]

(Extended) Describe two experiments, other than measuring pH, that would show hydrochloric acid is stronger than ethanoic acid of the same concentration. Give the expected results.

  • Add equal pieces of magnesium: hydrochloric acid fizzes faster. ✓
  • Measure electrical conductivity: hydrochloric acid conducts better (more ions). ✓
  • (Adding a carbonate is also acceptable: faster fizzing with hydrochloric acid.) ✓
05
Explain
[2 marks]

(Extended) Ammonia reacts with hydrogen chloride: NH3 + HCl → NH4Cl. Explain, in terms of protons, which substance is the acid and which is the base.

  • HCl donates a proton, so it is the acid. ✓
  • NH3 accepts the proton to form NH4+, so it is the base. ✓
06
Explain
[2 marks]

(Extended) Explain the difference between a dilute acid and a weak acid.

  • Dilute means a low concentration: little acid dissolved per dm3 of solution. ✓
  • Weak means the acid is only partially dissociated into ions, whatever its concentration. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.