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JEE Main levelSingle correct optionDifficulty 2 of 3mt027

In the cyanide leaching of gold ( $4 Au + 8 NaCN + O X_{2} + 2 H X_{2} O 4 Na [Au (CN) X_{2}] + 4 NaOH$ ), the function of the air bubbled through the pulp is to

Areduce the cyanide ion to $CN X^{2 -}$
Bneutralise the sodium hydroxide formed
Cproduce a stable froth
Doxidise gold to gold(I), which is then held in solution as the cyano complex

Answer: D

Gold does not dissolve in cyanide solution unless an oxidant is present. Oxygen takes electrons from

Au

(oxidation state

0 \to + 1

) and

CN X^{-}

stabilises

Au X^{+}

as

[Au (CN) X_{2}] X^{-}

. It does not make froth and it produces, not neutralises, hydroxide.

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