An aqueous solution contains `0.10 M H_(2)S` and 0.20 M HCl. If the equilibrium constants for the formation of `HS^(–)` from `H_(2)S` is `1.0xx10^(–7)` and that of `S^(2-)` from `HS^(–)` ions is `1.2xx10^(–13)` then the concentration of `S^(2-)` ions in aqueous solution is

Options

A. `5xx10^(-8)`
B. `3xx10^(-20)`
C. `6xx10^(-21)`
D. `5xx10^(-19)`

Answer

B

Explanation

In presence of external `H^(+)`,
`H_(2)ShArr2H^(+)+S^(2-),K_(a_(1)).K_(a_(2))=K_(eq)`
`:.([H^(+)]^(2)[S^(2-)])/([H]_(2)S)=1xx10^(-7)xx1.2xx10^(-13)`
`([0.2]^(2)[S^(2-)])/([0.1])=1.2xx10^(-20),[S^(2-)]=3xx10^(-20)`