JEE Main 2016 · Chemistry · d- and f-Block Elements and Metallurgy · medium

An ore sample of 40 g contains 62% Ag2S by mass. It is leached with aerated NaCN solution, Ag2S+4NaCN→ 2Na[Ag(CN)2]+Na2S, and the silver is then…

Single correct option

An ore sample of

40\ \mathrm{g}

contains

62\%

\mathrm{Ag_2S}

by mass. It is leached with aerated

\mathrm{NaCN}

solution,

\mathrm{Ag_2S+4NaCN\to 2Na[Ag(CN)_2]+Na_2S}

, and the silver is then displaced from the solution by zinc,

\mathrm{2Na[Ag(CN)_2]+Zn\to Na_2[Zn(CN)_4]+2Ag}

. With

\mathrm{Ag}=108

\mathrm{S}=32

\mathrm{Zn}=65.4

, the mass of zinc consumed for complete recovery is

Show answer and solution

Answer: (C) $6.54\ \mathrm{g}$

Mass of

\mathrm{Ag_2S}=0.62\times40=24.8

M=248

, so

0.100

mol. Each

\mathrm{Ag_2S}

gives 2 mol

\mathrm{Na[Ag(CN)_2]}

, i.e.

0.200

mol Ag. Zinc displaces silver in the ratio

\mathrm{Zn:Ag}=1:2

(Zn gives 2 electrons, each

\mathrm{Ag^+}

takes 1), so moles of Zn

=0.100

and mass

=0.100\times65.4=6.54

g. The

13.1

g option takes a

1:1

ratio,

3.27

g halves the amount, and

21.6

g is the mass of silver obtained.

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