2025 O Level Chem P1 Q24 — Electrolysis of Aqueous Solutions Solution

Answer: D. 'Equal volumes' of gas at the two electrodes means the moles of gas produced are the same at anode and cathode.

1) Dilute NaCl: at anode \(\text{O}_2\) forms (4 electrons per O₂); at cathode \(\text{H}_2\) forms (2 electrons per H₂). For the same charge passed, moles H₂ : moles O₂ = 2 : 1 — not equal.

2) Concentrated NaCl: anode produces \(\text{Cl}_2\) (2 electrons per Cl₂); cathode produces \(\text{H}_2\) (2 electrons per H₂). Same electrons per molecule → equal volumes. ✓

3) Molten NaCl: cathode produces Na metal (solid, not a gas). No gas at the cathode, so the question doesn't apply meaningfully — only one gas is produced.

Only experiment 2 gives equal gas volumes → D.