A buffer solution is prepared by mixing ‘x’ moles of $C{H}_{3}COONa$ and ‘y’ moles of $C{H}_{3}COOH$ such that (x + y) = 1, in water to make 1L buffer solution. If the buffer capacity of this buffer solution is plotted against moles of salt $C{H}_{3}COONa$ (with x) then the plot obtained will be (to scale approximately).

For an acidic buffer, buffer capacity is maximum when [salt] = [Acid].
In all graphs buffer capacity is maximum at [salt] = 0.5 i.e, it increases from 0.2 to 0.5
No. of moles of salt added = 0.3 (it is a basic salt)
$$\begin{gathered} pH=p^{Fa}+\log \frac{\left[S\right]}{\left[A\right]} \\ {p}^{H_1}=P^{Ka}+\log \frac{0.2}{0.8}\Rightarrow P^{H_1}=P^{Ka}-0.6 \\ {p}^{H_2}=P^{Ka}+\log \frac{0.5}{0.5}\Rightarrow P^{H_2}=P^{Ka} \\ \Delta pH=0.6 \\ \varphi =\frac{\text{No. of moles of acid/base added to 1L buffer}}{\text{change in pH}} \varphi =\frac{0.3}{0.6}=\frac{1}{2}=0.5 \end{gathered}$$