`K_(sp)` pf CdS os `8.0 xx 10^(-27)` and that of `H_(2)S` is `1 xx 10^(-22)`. A `1 xx 10^(-14)` M `CdCl_(2)` solution is precipitated on passing `H_(2)S` when pH is about

Options

A. 4
B. 6
C. 5
D. 7

Answer

C

Explanation

`H_(2)S hArr 2H^(+)+S^(2-)`
`CdShArr Cd^(2+)+S^(2-)`
`K_(sp) (CdS)=[Cd^(2+)][S^(2-)]`
`8 xx 10^(27)=(1 xx 10^(-14))[S^(2-)]`
`:. [S^(2-)]=8 xx 10^(-13)M`
`H_(s)S hArr 2H^(+)+S^(2-)`
`K_(sp)=[H^(+)]^(2)xx[S^(2-)]`
` 1 xx 10^(-22)=[H^(+)]^(2) xx8 xx 10^(-13)`
`[H^(+)]^(2)=(1xx10^(-22))/(8 xx 10^(-13))=(1)/(8) xx10^(-9)`
`=(10)/(8)xx10^(-10)`
`1.25 xx 10^(-10)`
`[H^(+)]=sqrt(1.25)xx10^(-5)`
`=1.12 xx 10^(-5)m`
`pH = - log [H^(+)]`
`= - log (1.12 xx 10^(-5))`
`=5- log 1.12`
`= 5- 0.0492`
`=4.9508~~5`