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Question

Find the concentration of H+ ion that must be maintained in a saturated solution of H2S (0.1 M) so as to precipitate CdS not ZnS
Given: [Cd2+]=[Zn2+]=(0.1 M) initially.
Ksp(CdS)=8×1027Ksp(ZnS)=1×1021Ksp(H2S)=1.1×1021

A
1.5 M
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B
0.5 M
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C
0.1 M
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D
0.95 M
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Solution

The correct option is C 0.1 M
In order to prevent precipitation of ZnS,
[Zn2+][S2]<Ksp(ZnS)or0.1×[S2]<1×1021[S2]<1×1020

This is the maximum value of [S2] before ZnS will precipitate.

H2S(s)2H+(aq)+S2(aq)Ka=[H+]2[S2][H2S]=x2×1×10200.1=1.1×1021x=[H+]=0.1 M
So, No ZnS will precipitate if concentration of H+ greater than 0.1 M.
So, this concentration of H+ must be maintained.

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