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

$H A^{-} \rightarrow H^{+}+A$

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

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

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

$=K_{1} \times K_{2}$

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

$=5 \times 10^{-15}$