The hydrogen ion concentration of 0.2N ${\mathrm{CH}}_3\mathrm{COOH}$,which is 40% dissociated is

${\mathrm{CH}}_3\mathrm{COOH}\rightleftharpoons {\mathrm{CH}}_3{\mathrm{COO}}^{-}+{\mathrm{H}}^{+}$

0.2                               0                       0

$0.2-\frac{0.2\times 40}{100} \frac{0.2\times 40}{100} \frac{0.2\times 40}{100}$

$\mathrm{C} {\mathrm{\alpha }}^{\mathrm{r}}=0.2\times \frac{40}{100}\times \frac{40}{100}$

$\left[{\mathrm{H}}^{+}\right]=\frac{0.2\times 40}{100}=\frac{0.2\times 4}{10}=0.08N$