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Question
Cu is precipitated as CuS while Zn is not precipitated when H2S is passed through an acidic solution of Cu(NO3 )2 and Zn(NO3)2 respectively.
Solution
This is because the Ksp of CuS is very low as compared to that of ZnS. Therefore, a low concentration of S2- ions can precipitate Cu2+ ions as CuS, whereas a high concentration of S2 ions is required to precipitate Zn2+ ions as ZnS.
Now, in the presence of HCl, dissociation of H2S is suppressed due to the common ion effect.
HCl → H+ + Cl– (strong electrolyte)
H2S → 2H+ + S2- (weak electrolyte)
With decreased S2- ion concentration only Cu2+ is precipitated as CuS.
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