Equilibrium constant for the dissolution of a compound.

• A. Equilibrium Constant
• B. Phase Diagram
• C. Osmotic Pressure
• D. Solubility Product Constant

Answer: (D)

When a solid dissolves in water, the relevant quantity is the solubility product constant, K_sp. For a sparingly soluble salt AB that dissolves as AB(s) ⇌ A+(aq) + B−(aq), K_sp is defined as the product of the activities of the ions: K_sp = a(A+) × a(B−). In dilute solutions, this is well approximated by the product of concentrations: K_sp ≈ [A+] × [B−]. This constant governs how much of the solid can dissolve before the system reaches equilibrium, and it depends on temperature.

This is a specific type of equilibrium constant tailored to dissolution. Phase diagrams relate to which phases are stable under conditions, while osmotic pressure concerns colligative properties, not the dissolution equilibrium constant. Hence, the term that best describes the equilibrium constant for the dissolution of a compound is the solubility product constant.