A solute that displays a strong common ion effect is AgCl dissolved in a solution containing Cl−. How does the common ion effect influence its solubility?

• A. Solubility is unaffected.
• B. Solubility decreases.
• C. Solubility increases.
• D. Solubility becomes zero.

Answer: (B)

The key idea is the common ion effect on a sparingly soluble salt. When AgCl dissolves, it establishes the equilibrium AgCl(s) ⇌ Ag+ + Cl−. The solubility product, Ksp, fixes the product [Ag+][Cl−]. If the solution already has a high concentration of Cl−, the equilibrium shifts to favor the solid, so less Ag+ (and thus less AgCl) dissolves. Since the solubility is essentially the concentration of Ag+ that can be present, increasing Cl− lowers [Ag+] and, therefore, lowers solubility. In mathematical terms, s ≈ Ksp/[Cl−], so a larger [Cl−] means a smaller solubility.