The concentration of hydrogen ions in a `0.2M` solution of formic acid is `6.4 xx 10^(-3) mol L^(-1)`. To this solution, sodium formate is added so as to adjust the concentration of sodium formate to `1 mol L^(-1)`. What will be the `pH` of this solution? The dissociation constant of formic acid is `2.4 xx 10^(-4)` and the degree of dissociation fo sodium formate is `0.75`.

Explanation

`HCOOH rarr H^(+) + HCOO^(-)`
`HCOONaunderset("1-0.75") rarr Na^(+) + HCOO^(-)underset("0.75")`
In the above buffer solution, the significant source of formate ion `(HCOO^(-))` is HCOONa. Hence,
`K_(a) = 2.4 xx 10^(-4) = [H^(+)](0.75)/[HCOOH]`
`[H^(+)] = 2.4 xx 10^(-4) xx 0.20/0.75 = 6.4 xx 10^(-5)`
pH = `=log(64 xx 10^(-5)) = 4.20`