Chemistry - Ionic Equilibrium Question with Solution | TestHub

$\begin{array}{ll}{\mathrm{H}}_2\mathrm{A}\rightleftharpoons {\mathrm{HA}}^{-}+{\mathrm{H}}^{+}& {\mathrm{K}}_1=\frac{\left[{\mathrm{HA}}^{-}\right]\left[{\mathrm{H}}^{+}\right]}{\left[{\mathrm{H}}_2\mathrm{A}\right]}\dots (1)\\ {\mathrm{HA}}^{-}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{A}}^{2-}& {\mathrm{K}}_2=\frac{\left[{\mathrm{H}}^{+}\right]\left[{\mathrm{A}}^{2-}\right]}{\left[{\mathrm{HA}}^{-}\right]}\dots (2)\end{array}$ For the reaction $\begin{array}{rl}& {\mathrm{H}}_2\mathrm{A}\rightleftharpoons 2{\mathrm{H}}^{+}+{\mathrm{A}}^{2-}\\ & \mathrm{K}=\frac{{\left[{\mathrm{H}}^{+}\right]}^2\left[{\mathrm{A}}^{2-}\right]}{\left[{\mathrm{H}}_2\mathrm{A}\right]}={\mathrm{K}}_1\times {\mathrm{K}}_2\\ & =1\times 10^{-5}\times 5\times 10^{-10}\\ & =5\times 10^{-15}\end{array}$ Hence, (C) is correct.