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An aqueous solution contains 0.10 M `H_(2)S " is "1.0 xx 10^(-7)" and that of " S^(2-) "ion from " HS^(-) "ions is " 1.2 xx 10^(-3) ," then the concentration of " S^(2-) " ions in aqueous solution is "`A. ` 5 xx 10^(-8)`B. `3 xx 10^(-20)`C. `6xx 10^(-21)`D. `5xx 10^(-19)`

Answer» Correct Answer - B
Given : (i) ` H_(2) S hArr HS^(-) + H^(+),`
`K_(a_(1)) = 1.0 xx 10^(-7)`
(ii) `HS^(-) hArr S^(2-) + H^(+), K_(a_(2))= 1.2 xx 10^(-13)`
For the reaction , ` H_(2)S hArr 2 H^(+) + S^(2-)`
(obtained by adding (i) and (ii) )
`K_(a) = K_(a_(1)) xx K_(a_(2))`
` :. (1.0 xx 10^(-7)) (1.2 xx 10^(-13))= ([H^(+)]^(2) [S^(2-)])/([H_(2)S]) `
In presence of HCl, `[H^(+)]` are obtained mainly from HCl. Hence ,
` [H^(+)] = 0.20 M, [H_(2) S]= 0.1 M `
`:. 1.2 xx 10^(-20) = ((0.2 )^(2)[S^(2-)])/0.1`
or `[S^(2-)] = 3 xx 10^(20)`


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