Which statement about the solubility product constant, Ksp, is correct?

Which statement about the solubility product constant, Ksp, is correct?

• A. Ksp is the rate constant for dissolution.
• B. Ksp is the solubility of a salt in water.
• C. Ksp is the equilibrium constant for dissolution of a sparingly soluble salt; precipitation occurs if Q > Ksp.
• D. Ksp equals the total ion concentration in solution.

Answer: (C)

Ksp is an equilibrium constant for the dissolution of a sparingly soluble salt. For a salt MX(s) that dissolves as MX(s) ⇌ M^n+(aq) + X^m−(aq), Ksp = a(M^n+) a(X^m−). In dilute solutions this is approximated by the product of the ion concentrations [M^n+][X^m−] (with the exponents matching the stoichiometry). Ksp describes how much solid can remain dissolved at a given temperature, not the rate of dissolution and not the total ion concentration. Precipitation occurs when the reaction quotient Q = [M^n+][X^m−] (or the appropriate product for the dissolution reaction) exceeds Ksp, because the solution is supersaturated and the system shifts toward forming more solid until Q returns to Ksp. If Q is less than Ksp, more solid can dissolve until equilibrium is reached; if Q equals Ksp, the system is at equilibrium with solid present. Remember, Ksp depends on temperature and is an indicator of the extent of dissolution at equilibrium, not a measure of solubility itself.

Ksp is an equilibrium constant for the dissolution of a sparingly soluble salt. For a salt MX(s) that dissolves as MX(s) ⇌ M^n+(aq) + X^m−(aq), Ksp = a(M^n+) a(X^m−). In dilute solutions this is approximated by the product of the ion concentrations [M^n+][X^m−] (with the exponents matching the stoichiometry). Ksp describes how much solid can remain dissolved at a given temperature, not the rate of dissolution and not the total ion concentration.

Precipitation occurs when the reaction quotient Q = [M^n+][X^m−] (or the appropriate product for the dissolution reaction) exceeds Ksp, because the solution is supersaturated and the system shifts toward forming more solid until Q returns to Ksp. If Q is less than Ksp, more solid can dissolve until equilibrium is reached; if Q equals Ksp, the system is at equilibrium with solid present. Remember, Ksp depends on temperature and is an indicator of the extent of dissolution at equilibrium, not a measure of solubility itself.