A Buffer solution having $\mathrm{pH} 4.72$ is prepared by mixing of $1\mathrm{M} \mathrm{NaOH}$ and $1\mathrm{M} \mathrm{HCN}$. What is the ratio of volume of acid to base? $[{\mathrm{pK}}_{\mathrm{b}} \mathrm{of} {\mathrm{CN}}^{-}= 9.28]$

$\mathrm{pOH} = {\mathrm{pK}}_{\mathrm{b}}+ \log \frac{\left(\mathrm{Salt}\right)}{\left(\mathrm{acid}\right)}$

$14 – 4.72 = 9.28 + \log \frac{\left(\mathrm{salt}\right)}{\left(\mathrm{acid}\right)}$

$\left(\mathrm{Salt}\right) = \left(\mathrm{acid}\right)$
it means half equivalence point.
or equivalents of base $=\frac{1}{2}$ {equivalents of acid}