b
Dissociation constant of weak acid \(=1 \times 10^{-4}\)

the \(\mathrm{pH}\) of weak acid \(=5\)

\(H A+H_{2} O \rightleftharpoons H_{3} O^{+}+A^{-}\)

\(K_{a}=\frac{\left[H_{3} O^{+}\right]\left[A^{-}\right]}{[H A]}\)

\(\left[H_{3} O^{+}\right]=\frac{K_{a} \times[H A]}{[A^-]}\)

\(p H=5=-\log \left[H_{3} O^{+}\right]\)\(=-\log \left[\frac{K_{0} \times[H A]}{[A-]}\right]=-\log K_{a}-\log \frac{[H A]}{[A]}\)

\(5=-\log \left(1 \times 10^{-4}\right)+\log \frac{\left[A^{-}\right]}{[H A]}\)

\(\frac{[A^-]}{[H A]}=10: 1\)