20 mL of 0.2 M NaOH is added to 50 mL of0.2M ${\mathrm{CH}}_3\mathrm{COOH}. \mathrm{Hence}$$\left({\mathrm{P}}^{\mathrm{H}}-{\mathrm{P}}^{{\mathrm{K}}_{\mathrm{a}}}\right) is$

${\mathrm{CH}}_3\mathrm{COOH}+\mathrm{NaOH}\rightleftharpoons {\mathrm{CH}}_3\mathrm{COONa}+{\mathrm{H}}_2\mathrm{O}$

0.01                    0.004                     0                        0 0.01−0.004        0                         0.004 0.006                      0                         0.004

$\mathrm{PH}-\mathrm{PKa}={\log }_{10}\left[\frac{\mathrm{Salt}}{\mathrm{Acid}}\right]$

$\mathrm{PH}-\mathrm{PKa}={\log }_{10}\frac{0.004}{0.006}={\log }_{10}^{2/3}$