JEE Main · 2022 · 25 Jun 2022 · Shift 1 · General Principles and Processes of Isolation of Metals

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Leaching of gold with dilute aqueous solution of NaCN in presence of oxygen gives complex [A], which on reaction with zinc forms the elemental gold and another complex [B]. [A] and [B], respectively are :-

(A) $[Au (CN)_{4}]^{-}$ and $[Zn (CN)_{2} (OH)_{2}]^{2 -}$
(B) $[Au (CN)_{2}]^{-}$ and $[Zn (OH)_{4}]^{2 -}$
(C) $[Au (CN)_{2}]^{-}$ and $[Zn (CN)_{4}]^{2 -}$ ✓ Correct
(D) $[Au (CN)_{4}]^{2 -}$ and $[Zn (CN)_{6}]^{4 -}$

Answer: (C)

Solution

The Mac Arthur-Forrest process (cyanide process) for gold extraction involves two steps:

Step 1: Gold is leached with dilute NaCN solution in presence of oxygen:

4 Au + 8 NaCN + O_{2} + 2 H_{2} O \to 4 Na [Au (CN)_{2}] + 4 NaOH

Complex [A] is

[Au (CN)_{2}]^{-}

Step 2: The gold complex reacts with zinc to precipitate gold:

2 Na [Au (CN)_{2}] + Zn \to Na_{2} [Zn (CN)_{4}] + 2 Au

Complex [B] is

[Zn (CN)_{4}]^{2 -}

Therefore, [A] and [B] are

[Au (CN)_{2}]^{-}

and

[Zn (CN)_{4}]^{2 -}

respectively.

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JEE Main · 2022 · 25 Jun 2022 · Shift 1 · General Principles and Processes of Isolation of Metals

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