c
\(\wedge_{ m }=\frac{ k }{ C } \times 1000\)

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

\(C =0.0025\,M\)

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

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

\(\alpha =\frac{20}{400}=\frac{1}{20}\)

\(\alpha=\frac{20}{400}=\frac{1}{20}\)

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

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

\(=66 \times 10^{-7}\)