Qualitative Analysis - Result Question 9
9. $CuSO _4$ decolourises on addition of $KCN$, the product is
(2006, 3M)
(a) $\left[Cu(CN) _4\right]^{2-}$
(b) $Cu^{2+}$ get reduced to form $\left[Cu(CN) _4\right]^{3-}$
(c) $Cu(CN) _2$
(d) $CuCN$
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Answer:
Correct Answer: 9. (b)
Solution:
$CuSO _4+2 KCN \longrightarrow Cu(CN) _2+K _2 SO _4$
$\begin{aligned} & \underset{\text { Unstable }}{2 Cu(CN) _2} \longrightarrow 2 Cu(CN)+ NC-CN(\text { Cyanogen }) \\ & CuCN+3 KCN \longrightarrow 3 K^{+}+\left[Cu(CN) _4\right]^{3-} \end{aligned}$
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