The `[H^(+)]` is `2.1xx10^(-3)M` in a `0.072M` solution of benzoic acid. Calculate `K_(a)` for benzoic acid.

Explanation

`{:(C_(6)H_(5)COOHhArr,C_(6)H_(5)COO^(-)+,H^(+)),(1,0,0),((1-alpha),alpha,alpha):}`
`[H^(+)]=Calpha,[C_(6)H_(5)COO^(-)]=Calpha`,
`[C_(6H_(5)COOH]=C(1-alpha)`
`("Given": Calpha=2.1xx10^(-3)M)`
`:. K_(a)=([H^(+)][C_(6)H_(5)COO^(-)])/([C_(6)H_(5)COOH])=(Calpha.Calpha)/(C(1-alpha))`
`=(2.1xx10^(-3)xx2.1xx10^(-3))/((0.072-0.002))=6.3xx10^(-5)`