20 millimoles of HCl added to one litre of  Buffer showed a ${\mathrm{p}}^{\mathrm{H}}$ change of 0.4 units. Buffer capacity of the buffer is

$$\begin{gathered} \mathrm{Buffer} \mathrm{capacity}\left(\mathrm{\varphi }\right)=\frac{\mathrm{Number} \mathrm{gram} \mathrm{equivalents} \mathrm{of} \mathrm{acid} \mathrm{or} \mathrm{base} \mathrm{added} \mathrm{per} \mathrm{litre} \mathrm{of} \mathrm{buffer}}{\mathrm{change} \mathrm{in} {\mathrm{p}}^{\mathrm{H}}}; \\ \mathrm{Buffer} \mathrm{capacity}\left(\mathrm{\varphi }\right)=\frac{20\times {10}^{-3}}{0.4}=0.05; \end{gathered}$$