`100 mL` of `0.02M` benzoic acid `(pK_(a)=4.2)` is titrated using `0.02 M NaOH`. `pH` values after `50 mL` and `100 mL` of `NaOH` have been added are

Options

A. `3.50,7`
B. `4.2,7`
C. `4.2,8.1`
D. `4.2,8.25`

Answer

C

Explanation

`underset(t_(eq))underset(t=0)(C_(6)H_(5)COOH)+underset(1)underset(2)(OH^(-)) rarr underset(-)underset(1)(C_(6)H_(5)COO^(-))+ underset(1)underset()(H_(2)O)`
`pH=pK_(a)=4.2`
`underset(t_(eq))underset(t=0)(C_(6)H_(5)COOH)+underset(-)underset(2)(OH^(-)) rarr underset(-)underset(2)(C_(6)H_(5)COO^(-))+ underset(2)underset()(H_(2)O)`
`[C_(6)H_(5)COO^(-)]=(2)/(200)=0.01 M`
`:. pH=7+pK_(a)+(1)/(2)logC`
`=7+(4.2)/(2)+(1)/(2)log(0.01)=8.1`