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Question

H2S is bubbled into 0.2M NaCN solution which is 0.02 M in each [Cd(CN)4]2and[Ag(CN)2]. The H2S produces 1×109 M sulphide in the solution. (Given, KspAg2S=1×1050M3;KspCdS=7.1×1028M2Kinst[Ag(CN)2]1=1×1020M2Kinst[Cd(CN)4]2=7.8×1018M4) Identify the correct statement


A

Ag2S precipitates first from the solution

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B

CdS precipitates first from the solution

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C

None of them precipitate under the given conditions

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D

Ag2S precipitates at a sulphide concentration of 1×1015M

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Solution

The correct option is B

CdS precipitates first from the solution


Ag(CN)2Ag++2CNKinst=1×1020M2Cd(CN)24Cd+2+4CNKinst=7.8×1018M4[Ag+]=Kinst×Ag(CN2)[CN]2=1×1020×0.02(0.2)2=5×1021M
Similarly, [Cd+2]=7.8×1018×0.02(0.2)4M=9.75×1017M
[S2] needed to precipitate [Ag+]=KSP×[Ag2S][Ag+]21×1050(5×1021)2=4×1010M
Since [S2] needed to precipitate Cd+2 ion is smaller than that of Ag+ ion.
Therefore Cd+2 will precipitate first.


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