2025 H2 Chem P1 Q29 — Electrolysis Solution

The answer is B.
Let the charge on the copper ion be \(z\), so \(\text{Cu}^{z+} + z\text{e}^- \rightarrow \text{Cu}\).
• Charge passed: \(Q = It = X \times 2 \times 60 \times 60\) C (2 hours in seconds).
• Moles of electrons: \(n(\text{e}^-) = \dfrac{X \times 2 \times 60 \times 60}{96500}\)
• Moles of copper: \(n(\text{Cu}) = \dfrac{Y}{63.5}\)
\(z = \dfrac{n(\text{e}^-)}{n(\text{Cu})} = \dfrac{X \times 2 \times 60 \times 60 \times 63.5}{96500 \times Y}\)
• A is wrong: it uses \(2 \times 60\), which is the time in minutes, not seconds.
• C is wrong: it multiplies \(n(\text{e}^-)\) by \(n(\text{Cu}) = \dfrac{Y}{63.5}\) instead of dividing by it.
• D is wrong: it is the reciprocal of the correct expression, \(\dfrac{n(\text{Cu})}{n(\text{e}^-)}\).