Gold is also extracted by cyanide process as in case of silver.

Write the reactions.

2 [A u (C N)_{2}]^{-} + Z n \to [Z n (C N)_{4}]^{2 -} + 2 A u ↓

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4 [A u (C N)_{2}]^{-} + Z n \to [Z n (C N)_{4}]^{2 -} + 4 A u ↓

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[A u (C N)_{2}]^{-} + Z n \to [Z n (C N)_{4}]^{2 -} + A u ↓

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None of these

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Solution

The correct option is A $2 [A u (C N)_{2}]^{-} + Z n \to [Z n (C N)_{4}]^{2 -} + 2 A u ↓$
In the cyanide process, crushed rock containing traces of gold is treated with $0.1 - 0.2 %$ solution of NaCN and aerated. $O_{2}$ (air) oxidises free metal to $A u^{+}$ which complexes with $C N^{-}$

$4 A u + 8 C N^{-} + 2 H_{2} O + O_{2} \to 4 [A u (C N) 2]^{-} + 4 O H^{-}$

The pure metal is displaced from solution by an active metal. $2 [A u (C N)_{2}]^{-} + Z n \to [Z n (C N)_{4}]^{2 -} + 2 A u ↓$

So, the correct option is (A)

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