In equalitative analysis, cations of graph `II` as well as group `IV` both are precipitated in the form of sulphides. Due to low value of `K_(sp)` of group `II` sulphides, group reagent is `H_(2)S` in the presence of dil. `HC1`, and due to high value of `K_(sp)` of group `IV` sulphides, group reagent is `H_(2)S` in the presence of `NH_(4)OH` and `NH_(4)C1`. In a solution containing `0.1M` each of `Sn^(2+), Cd^(2+)`, and `Ni^(2+)` ions, `H_(2)S`gas is passed.
`K_(sp) of SnS = 8 xx 10^(-29), K_(sp) of CdS = 15 10^(-28), K_(sp) of NiS - 3 xx 10^(-21), K_(1) of H_(2)S = 1 xx 10^(-7), K_(2) of H_(2)S = 1 xx 10^(-14)`
If `0.1M HC1` is mixed in the solution containing only `0.1 M Cd^(2+)` ions and saturated with `H_(2)S`, then `[Cd^(2+)]` remaining in the solution after `CdS` stopes to precipitate is:

Options

A. `10^(-8)`
B. `8.2 xx 10^(-9)`
C. `5.6 xx 10^(-6)`
D. `5.6 xx 10^(-10)`

Answer

A

Explanation

`K_(1)K_(2) = ([H^(o+)]^(2)[S^(2-)])/([H^(2)S]) rArr [S^(2-)]_(min) = 10^(-20)M`
`rArr [Cd^(2+)]_("allowed") = (K_(sp)(CdS))/([S^(2-)]_("salt")) = 10^(-8)M`