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An acidic aqueous solution contains CuSO4, ZnSO4, MgSO4, and CdSO4. When H2S(g) is passed through it, which ion(s) are precipitated as metal sulphides?

1. Cu2+, Cd2+
2. only Cu2+
3. Mg2+, Zn2+
4. Cu2+, Zn2+, Cd2+

Correct answer: Cu2+, Cd2+

Solution

In acidic medium, [S2-] is suppressed. CuS (Ksp ~ 10⁻³⁶) and CdS (Ksp ~ 10⁻²⁸) precipitate. ZnS and MgS do not precipitate in acid.

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