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A metal cation M²+ forms a black precipitate with acidified H2S that dissolves in hot concentrated HNO3. An aqueous solution of M²+ gives a brown precipitate on treatment with potassium ferrocyanide (K4[Fe(CN)6]). Which of the following statements are correct?

1. M²+(aq) reacts with excess KCN to release a gas.
2. Metal M is obtained from its most common sulphide ore via an auto-reduction (self-reduction) process.
3. The standard electrode potential E0(M²+/M) is positive.
4. M²+(aq) produces a dark blue solution with excess Cl⁻ ions.

Correct answer: M²+(aq) reacts with excess KCN to release a gas.

Solution

M is copper (Cu). CuS is black, dissolves in hot conc. HNO3. Cu²+ gives reddish-brown precipitate with K4[Fe(CN)6]. A: Cu²+ + KCN(excess) → [Cu(CN)4]³- + (CN)2 gas — correct. B: Cu is extracted from copper pyrites by auto-reduction (2Cu2O + Cu2S → 6Cu + SO2) — correct. C: E0(Cu²+/Cu) = +0.34 V > 0 — correct. D: Deep blue colour with excess NH3 (not Cl⁻) — incorrect.

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