For a salt AB(s) dissolving into A+ and B-, what is the expression for Ksp?

For a salt AB(s) dissolving into A+ and B-, what is the expression for Ksp?

• A. [A+]/[B-]
• B. [A+][B-]
• C. [A-][B+]
• D. [A+]-[B-]

Answer: (B)

The key idea is that the solubility product constant for a sparingly soluble salt comes from the equilibrium between the solid and its dissolved ions, and it is the product of the ion concentrations at equilibrium. For AB(s) dissolving, the dissolution is AB(s) ⇌ A+(aq) + B−(aq). The Ksp expression uses the activities of the ions, and because the coefficients are 1:1, the expression is Ksp = a_A+ a_B−. In dilute solutions, activities are well approximated by concentrations, so Ksp ≈ [A+][B−]. The product form matters because it reflects the simultaneous presence of both ions at equilibrium; a ratio or a difference would not describe this dissolution equilibrium. The ions involved are A+ and B−, not A− or B+.

The key idea is that the solubility product constant for a sparingly soluble salt comes from the equilibrium between the solid and its dissolved ions, and it is the product of the ion concentrations at equilibrium. For AB(s) dissolving, the dissolution is AB(s) ⇌ A+(aq) + B−(aq). The Ksp expression uses the activities of the ions, and because the coefficients are 1:1, the expression is Ksp = a_A+ a_B−. In dilute solutions, activities are well approximated by concentrations, so Ksp ≈ [A+][B−]. The product form matters because it reflects the simultaneous presence of both ions at equilibrium; a ratio or a difference would not describe this dissolution equilibrium. The ions involved are A+ and B−, not A− or B+.