Chemistry - Ionic Equilibrium Question with Solution | TestHub

For acidic buffer pH can be find out as follows
$\text{pH}=\text{pKa}+\text{lo}{\text{g}}_{10}\frac{\left[\mathrm{s}\mathrm{a}\mathrm{l}\mathrm{t}\right]}{\left[\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}\right]}$
As here${\mathrm{K}}_{\mathrm{b}}=10^{-9}$
So${\mathrm{K}}_{\mathrm{a}}=10^{-5}$
$\text{p}{\text{K}}_{\mathrm{a}}=5$
As$\text{pH}=5+\mathrm{l}\mathrm{o}\mathrm{g}{}_{10}\frac{\left[{\mathrm{B}}^{-}\right]}{\left[\mathrm{H}\mathrm{B}\right]}$
As$\left[{\mathrm{B}}^{-}\right]=\left[\text{HB}\right]$
$\text{pH}=5+\text{log}\text{1}=\text{5}\text{+}\text{0}\text{=}\text{5}$