The ${\mathrm{K}}_{\mathrm{a}}$ for formic acid  and  acetic acid are $2 \times {10}^{-4}$ and $2 \times {10}^{-5}$ respectively. Calculate the relative strength of acids with same molar concentration

 $\left[{\mathrm{H}}^{+}\right] =\sqrt{{\mathrm{K}}_{\mathrm{a}}\mathrm{C}}$

 Relative strength of weak acids = $\sqrt{\left(\frac{{\mathrm{K}}_{{\mathrm{a}}_1}}{{\mathrm{K}}_{{\mathrm{a}}_2}} \times \frac{{\mathrm{C}}_1}{{\mathrm{C}}_2}\right)}$

$\therefore$ Relative strength = $\sqrt{\left(\frac{{\mathrm{K}}_{\mathrm{a}1}}{{\mathrm{K}}_{\mathrm{a}2}} \right)}$              $\left(\because C_1 =C_2\right)$

                             $=\sqrt{\left(\frac{2 \times {10}^{-4}}{2 \times {10}^{-5}} \right)}$

Relative strength for HCOOH to ${\mathrm{CH}}_3\mathrm{COOH} = \sqrt{10} :\hspace{0.17em}1$