A student compares two acids: hydrochloric acid and methanoic acid, both at a concentration of 0.01 mol/dm³. Explain why the electrical conductivity of the hydrochloric acid solution is greater than that of the methanoic acid solution at the same concentration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrochloric acid is a strong acid. Methanoic acid is a weak acid.
Model answer (5 marks)
Hydrochloric acid is a strong acid, so it dissociates completely in water:
HCl → H⁺ + Cl⁻
Thus a 0.01 mol dm⁻³ solution contains 0.01 mol dm⁻³ of H⁺ and 0.01 mol dm⁻³ of Cl⁻.
Methanoic acid is a weak acid and only partially dissociates:
CH₃COOH ⇌ H⁺ + CH₃COO⁻
At the same concentration the equilibrium lies far to the left, giving a much lower concentration of H⁺ and CH₃COO⁻.
Electrical conductivity is proportional to the concentration of charge‑carrying ions. Because the HCl solution contains a higher concentration of ions, its conductivity is greater than that of the methanoic acid solution.
HCl → H⁺ + Cl⁻
Thus a 0.01 mol dm⁻³ solution contains 0.01 mol dm⁻³ of H⁺ and 0.01 mol dm⁻³ of Cl⁻.
Methanoic acid is a weak acid and only partially dissociates:
CH₃COOH ⇌ H⁺ + CH₃COO⁻
At the same concentration the equilibrium lies far to the left, giving a much lower concentration of H⁺ and CH₃COO⁻.
Electrical conductivity is proportional to the concentration of charge‑carrying ions. Because the HCl solution contains a higher concentration of ions, its conductivity is greater than that of the methanoic acid solution.
Examiner tips
- State that strong acids fully dissociate, weak acids only partially.
- Show the ion concentrations for each acid at 0.01 mol dm⁻³.
- Explain that conductivity depends on ion concentration and mobility.
- Link the higher ion concentration in HCl to its higher conductivity.
Common mistakes
- Confusing concentration of the acid with concentration of ions; not recognising partial dissociation of methanoic acid.
- Using the wrong ion for methanoic acid (e.g., CH₃COOH instead of CH₃COO⁻).
- Failing to mention that conductivity is proportional to ion concentration.
Mark scheme (5 marks)
- Strong acids fully/completely dissociate (in water)
- Weak acids only partially dissociate (in water), establishing an equilibrium
- Hydrochloric acid produces a greater concentration of ions / more ions in solution than methanoic acid at the same concentration
- Electrical conductivity depends on the concentration / number of ions (that can carry charge / current)
- Therefore hydrochloric acid has higher conductivity because it has a greater concentration of ions than methanoic acid at the same concentration
Key terms in this question
electrical conductivity · concentration
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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