Which of the following is true

Options

A. `pK_(b)` for `OH^(-)` is `-1.74` at `25^(@)C`
B. The equilibrium constant for the reaction between `HA (pK_(a)=4)` and `NaOH` at `25^(@)C` will be equal to `10^(10)`.
C. The `pH` of a solution containing `0.1 M HCOOH`
`(K_(a)=1.8xx10^(-4))` and `0.1 M HOCN`
`(K_(a)=3.2xx10^(-4))` will be nearly `(3-log 7)`.
D. all the above are correct.

Answer

C

Explanation

`(C )[H^(+)]=sqrt(K_(a_(1))C_(1)+K_(a_(2))C_(2))=sqrt(5xx10^(-5))`
`pH=(5-log5)/(2)=3-log7`