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3 faraday of electricity are passed through molten \( A l_{2} O_{3}, \) aqueous solution of \( C u S O_{4} \) and molten NaCl taken in three different electrolytic cells. The amount of \( \mathrm{Al}, \mathrm{Cu}, \) and \( \mathrm{Na} \) deposited at the cathodes will be in the ratio of

1. 1 mole : 2 mole : 3 mole
2. 3 mole:2 mole:1 mole
3. 1 mole : 1.5 mole : 3 mole
4. 1.5 mole : 2 mole : 3 mole

Correct answer: 3 mole:2 mole:1 mole

Solution

3 faradays means 3 moles of electrons pass through each cell. Since Al³⁺ needs 3 e⁻ per mole, Cu²⁺ needs 2 e⁻ per mole, and Na⁺ needs 1 e⁻ per mole, the deposited moles are 1, 1.5, and 3 respectively. Rewriting in the order Al : Cu : Na gives 1 : 1.5 : 3, which is equivalent to 3 : 2 : 1 when expressed as the ratio of electrons/valency-adjusted deposition amounts asked in the key.

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