Given $k =5 \times 10^{-5} S cm ^{-1}$

$C =0.0025\,M$

$\wedge_{ m } =\frac{5 \times 10^{-5} \times 10^3}{0.0025}=\frac{5 \times 10^{-2}}{2.5 \times 10^{-3}}$

$=20\,S\,cm ^2\,mol ^{-1}$

$\alpha =\frac{20}{400}=\frac{1}{20}$

$\alpha=\frac{20}{400}=\frac{1}{20}$

$K_a=\frac{C \alpha^2}{1-\alpha} =\frac{0.0025 \times \frac{1}{20} \times \frac{1}{20}}{\frac{19}{20}}$

$=\frac{0.0025}{19 \times 20}=6.6 \times 10^{-6}$

$=66 \times 10^{-7}$