Which statement about the solubility product constant (Ksp) is correct?

Which statement about the solubility product constant (Ksp) is correct?

• A. Higher Ksp means less solubility.
• B. Higher Ksp means more solubility.
• C. Ksp only applies to gases.
• D. Ksp equals the maximum solubility of the salt in solution.

Answer: (B)

Solubility product constant, Ksp, comes from the dissolution equilibrium of a salt into its ions. For a salt that dissolves as one cation and one anion, the equilibrium is Salt(s) ⇌ A+(aq) + B−(aq), and Ksp = [A+][B−]. If the salt’s molar solubility is s, then [A+] = s and [B−] = s, so Ksp = s^2. This means that when Ksp is larger, the solution can reach a higher product of ion concentrations before precipitation occurs, which corresponds to a higher molar solubility. In other words, a bigger Ksp indicates more salt can dissolve. The other statements don’t fit because Ksp does not indicate solubility of gases (that’s a different concept, Henry’s law), and Ksp is not the numerical value of maximum solubility itself but the equilibrium constraint that, for a given temperature, ties together ion concentrations. For common 1:1 salts, solubility scales with the square root of Ksp, so a larger Ksp indeed means greater solubility.

Solubility product constant, Ksp, comes from the dissolution equilibrium of a salt into its ions. For a salt that dissolves as one cation and one anion, the equilibrium is Salt(s) ⇌ A+(aq) + B−(aq), and Ksp = [A+][B−]. If the salt’s molar solubility is s, then [A+] = s and [B−] = s, so Ksp = s^2. This means that when Ksp is larger, the solution can reach a higher product of ion concentrations before precipitation occurs, which corresponds to a higher molar solubility. In other words, a bigger Ksp indicates more salt can dissolve.

The other statements don’t fit because Ksp does not indicate solubility of gases (that’s a different concept, Henry’s law), and Ksp is not the numerical value of maximum solubility itself but the equilibrium constraint that, for a given temperature, ties together ion concentrations. For common 1:1 salts, solubility scales with the square root of Ksp, so a larger Ksp indeed means greater solubility.