d
\(H_{2} A \rightleftharpoons H^{+}+H A\)

\(\therefore K_{1}=1.0 \times 10^{-5}=\frac{\left[H^{+} \| H A^{-}\right]}{\left[H_{2} A\right]}(\text { Given })\)

\(H A^{-} \rightarrow H^{+}+A\)

\(\therefore K_{2}=5.0 \times 10^{-10}\)

\(=\frac{| H^{+} \| A^{-1}}{\left|H A^{-}\right|}(\text {Given })\)

\(K=\frac{\left[H^{+}\right]^{2}\left[A^{2}\right]}{\left[H_{2} A\right]}\)

\(=K_{1} \times K_{2}\)

\(=\left(1.0 \times 10^{-5}\right) \times\left(5 \times 10^{-10}\right)\)

\(=5 \times 10^{-15}\)