The value of K_{sp} for BaCrO_{4}(s) in equilibrium with an aqueous solution of its constituent ions at 25°C is 1.2 × 10^{−10}\ M^{2}. Write the chemical equation that represents the solubility equilibrium for BaCrO_{4}(s) and calculate its solubility in water in grams per liter at 25°C.
The chemical equation that describes the solubility equilibrium is
BaCrO_{4}(s) ⇋ Ba^{2+}(aq) + CrO_{4}^{2−}(aq)The K_{sp} expression for this equation is
K_{sp} = [Ba^{2+}][CrO_{4}^{2−}] = 1.2 × 10^{–10}\ M^{2}If BaCrO_{4}(s) equilibrated with pure water, then, from the reaction stoichiometry, we have at equilibrium
[Ba^{2+}] = [CrO_{4}^{2−}] = swhere s is the solubility of BaCrO_{4}(s) in pure water. Thus,
K_{sp} = s^{2} = 1.2 × 10^{–10}\ M^{2}and
s= (1.2 × 10^{–10} M^{2})^{1⁄ 2} = 1.1 × 10^{–5}\ MThe solubility in grams per liter is given by
s = (1.1 × 10^{–5}\ mol·L^{–1}) \left(\frac{253.3\ g\ BaCrO_{4}}{1\ mol\ BaCrO_{4}} \right) = 2.8 × 10^{–3}\ g·L^{–1}