QUESTION 139 (5 marks)
A copper-plating cell uses an inert anode. The current is held constant during each interval in the table. Assume Cu²⁺ reduction at the cathode and oxygen formation from water at the anode, each with 100% current efficiency. Use F = 96485 C mol⁻¹, M(Cu) = 63.55 g mol⁻¹ and gas molar volume 24.8 L mol⁻¹.
| Interval / s | Current / A |
|---|---|
| 0–300 | 1.20 |
| 300–900 | 0.400 |
(a) Calculate the total charge passed. [1 marks]
(b) Calculate the mass of copper deposited. [2 marks]
(c) Calculate the volume of oxygen produced in millilitres. [2 marks]
Practice marking scheme
Answer
Total charge = 600 C; copper mass = 0.198 g; oxygen volume = 38.6 mL.
Working
Sum for the intervals: C. The copper amount is and its mass is g. Water oxidation releases four electrons per O₂, so mol. The volume is mL.
Marking criteria
- Sums the interval charges to obtain 600 C. [1 mark]
- Uses n(Cu) = q/(2F). [1 mark]
- Calculates copper mass ≈ 0.198 g. [1 mark]
- Uses n(O₂) = q/(4F). [1 mark]
- Calculates oxygen volume ≈ 38.6 mL. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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