The solubility in water of a sparingly soluble salt AB₂ is 1.0 x 10^-5 mol L^-1. Its solubility product Will

$\underset{1}{{\mathrm{AB}}_2}\to \underset{\mathrm{S}}{{\mathrm{A}}_{ }^{2+}}+\underset{2\mathrm{S}}{2{\mathrm{B}}^{-}}$

By the relation of solubility product and solubility,

$\begin{array}{l}{\mathrm{K}}_{\mathrm{sp}}=\left[{\mathrm{A}}^{2+}\right]{\left[{\mathrm{B}}^{-}\right]}^2\\ {\mathrm{K}}_{\mathrm{sp}}=\mathrm{S}\cdot (2\mathrm{S}{)}^2\end{array}$

After putting S=10⁻⁵ M,

$\begin{array}{l}{\mathrm{K}}_{\mathrm{sp}}={10}^{-5}\times {\left(2\times {10}^{-5}\right)}^2\\ {\mathrm{K}}_{\mathrm{sp}}=4\times {10}^{-15}\end{array}$