Ksp and Solubility

Strontium chromate SrCrO₄ dissolves in water to form the ions Sr²⁺ and CrO₄^2-. Each mole of the salt dissolves to form a mole of Sr²⁺ and a mole of CrO₄. A mole of sodium carbonate Na₂CO₃ dissolves to form 2 moles of Na⁺ and 1 mole of CO₃^2-. So if the solubility of Na₂CO₃ is s moles per liter the solution will contain 2s moles of Na⁺ and s moles of CO₃^2-. This gives the relation between solubility in moles per liter and the solubility product. Given the solubility product of CaCO₃ as 4.9 x 10^-9, we can calculate the solubility in water. CaCO₃ will ionize to form Ca²⁺ and CO₃^2-. There will be equal concentrations of both. So the solubility product will be given by [Ca²⁺] x [CO₃^2-] which is equal to [Ca²⁺]². This gives [Ca²⁺]² = 4.9 x 10^-9. Thus the concentration of Ca²⁺ is the square root of K_sp or 7.0 x 10^-5. Since each mole of the salt produces 1 mole of Ca⁺ the solubility of the salt is 7.0 x 10^-5 M.