When a solution of benzoic acid was titrated with `NaOH` the `pH` of the solution when half the acid neutralized was `4.2`. Dissociation constant of the acid is

Options

A. `6.31xx10^(-5)`
B. `3.2xx10^(-5)`
C. `8.7xx10^(-8)`
D. `6.42xx10^(-4)`

Answer

A

Explanation

`underset(After neutralization)(C_(6)H_(5)COOH)+NaOHhArrunderset(0.5)(C_(6)H_(5)COONa)+underset(0.5)(H_(2)O)`
It is a buffer solution of weak acid and its salt
`pH=pK_(a)+log``(["salt"])/([acid])`
`pK_(a)=4.2`
`K_(a)=6.31xx10^(-5)`