A sparingly soluble electrolyte $\mathrm{M}_{2} \mathrm{X}$ ionizes as $\mathrm{M}_{2} \mathrm{X} \rightleftharpoons 2 \mathrm{M}^{+}+\mathrm{X}^{2-}$. The solubility product $\left(\mathrm{K}_{\mathrm{sp}}\right)$, molal solubility $(\mathrm{S})$ and molal activity coefficient $\left(\gamma_{ \pm}\right)$are related by