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An aqueous solution of a transition metal sulphate reacts with gas to give a black precipitate. ThisQualitative and Quantitative Analysis Chemistry Question

Question

An aqueous solution of a transition metal sulphate reacts with $H_2S$ gas to give a black precipitate. This precipitate is filtered and found to be soluble in 50% $HNO_3$. Treating this resulting solution with an excess of $NH_4OH$ yields an intense deep-blue solution. The transition metal is:

Answer: C

💡 Solution & Explanation

The black precipitate soluble in 50% $HNO_3$ points to $CuS$ (unlike $HgS$, $CoS$, or $NiS$ which require aqua regia). The definitive confirmation is the intense deep-blue solution formed with excess ammonia, which is the tetraamminecopper(II) complex, $[Cu(NH_3)_4]^{2+}$.

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