$0.1\ \mathrm{mol}$ of sodium formate was added to one litre of 0.2 M solution of formic acid ($K_\mathrm a=1.8\times10^{-4}$) how much will the $\ce{H+}$ concentration of the acid diminish assuming the salt to be completely dissociated ?
My attempt :
Found out the $\ce{[H+]}$in formic acid.
Now for calculating $\ce{[H+]}$ in $\ce{HCOONa}$, I am planning to use the equation: $$\mathrm{pH}=\mathrm pK_\mathrm a+\log\frac{[\text{salt}]}{[\text{acid}]}$$
But I don't know the $\mathrm pK_\mathrm a$ of $\ce{HCOONa}$
How do I proceed , any alternative approaches ?