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:

  1. $CuSO _4+2 KCN \longrightarrow Cu(CN) _2+K _2 SO _4$

$$ \begin{aligned} & \underset{\text { Unstable }}{2 Cu(CN) _2} \longrightarrow 2 Cu(CN)+\left.\right| _{CN} ^{CN}(\text { Cyanogen }) \\ & CuCN+3 KCN \longrightarrow 3 K^{+}+\left[Cu(CN) _4\right]^{3-} \end{aligned} $$



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