What is maximum pH required to prevent the precipitation of ZnS in a solution that is 0.01 M `ZnCl_(2)` and saturated with 0.10M `H_(2)S`?
[Given : `K_(sp)(ZnS)=10^(-21),`
`K_(a_(1))xxK_(a_(2))` (of `H_(2)S`)=`10^(-20)`]

Options

A. `0`
B. `1`
C. `2`
D. `4`

Answer

B

Explanation

`K_(sp) = [Zn^(2+)][S^(2-)]`
`[S^(2-)] = (10^(-21))/(0.01) = 10^(-19)`
for `K_(a_(1)), K_(a_(2)) = ([H^(+)]^(2)[S^(2-)])/([H_(2)S])`
`10^(-20) = ([H^(+)]^(2) xx 10^(-19))/(0.1) rArr [H^(+)] = 0.1`
or `pH = 1`