The first and second dissociation constants of an acid `H_2A` are `1.0 xx 10^(-5)` and `5.0 xx 10^(-10)` respectively. The overall dissociation constant of the acid will be

Options

A. `0.2xx10^5`
B. `5.0xx10^(-5)`
C. `5.0xx10^15`
D. `5.0xx10^(-15)`

Answer

D

Explanation

`H_2A hArr H^(+) + HA^(-)` ,
`K_1=([H^+][A^-])/([H_2A])=1xx10^(-5)`
`HA^(-) hArr H^(+) + A^(2-) , K_2=5xx10^(-10) ([H^+][A^(2-)])/([HA^-])`
`K=([H^+][A^(2-)])/([H_2A])=K_1xxK_2`
`=1xx10^(-5)xx5xx10^(-10)`
`=5xx10^(-15)` .