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JEE Main 2020 · Chemistry · d- and f-Block Elements and Metallurgy · medium

In the cyanide process, argentite is leached with aerated NaCN to give Na[Ag(CN)2], from which silver is displaced by zinc…

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In the cyanide process, argentite is leached with aerated NaCN\mathrm{NaCN} to give Na[Ag(CN)2]\mathrm{Na[Ag(CN)_2]}, from which silver is displaced by zinc: Zn+2[Ag(CN)2]−→2Ag+[Zn(CN)4]2−\mathrm{Zn+2[Ag(CN)_2]^-\rightarrow2Ag+[Zn(CN)_4]^{2-}}. A leach solution holds 2.16 g2.16\ \mathrm{g} of silver as the complex. The minimum mass of zinc needed (Ag=108\mathrm{Ag}=108, Zn=65\mathrm{Zn}=65) is
  1. A. 0.65 g0.65\ \mathrm{g}
  2. B. 1.30 g1.30\ \mathrm{g}
  3. C. 0.325 g0.325\ \mathrm{g}
  4. D. 2.16 g2.16\ \mathrm{g}
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Answer: (A) 0.65 g0.65\ \mathrm{g}

Moles of Ag =2.16/108=0.020=2.16/108=0.020. Each Zn displaces 22 Ag, so Zn needed =0.010=0.010 mol =0.010×65=0.65=0.010\times65=0.65 g. Air in the leaching step oxidises the S2−\mathrm{S^{2-}} formed to sulphate, pushing the leaching equilibrium forward; zinc is the reducing agent in the second step.
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