Define Ksp and explain how to determine if a salt will precipitate.

Define Ksp and explain how to determine if a salt will precipitate.

• A. Precipitation occurs when Q > Ksp.
• B. Precipitation occurs when Q < Ksp.
• C. Precipitation occurs when Q equals Ksp.
• D. Precipitation occurs independent of Q and Ksp.

Answer: (A)

Ksp tells you how much of a sparingly soluble salt can dissolve before the solution becomes saturated. For a salt that dissociates into ions, Ksp is the product of the ion concentrations at equilibrium (each raised to its stoichiometric power). When you mix solutions, you compute Q, the ion product, using the actual concentrations present: if Q > Ksp, the solution is supersaturated with respect to that salt, and excess ions will come out of solution as solid to bring Q down to Ksp. If Q < Ksp, dissolution (if solid is present) can continue until Q reaches Ksp; if Q = Ksp, the system is at equilibrium with no net precipitation. Therefore, precipitation occurs when Q exceeds Ksp.

Ksp tells you how much of a sparingly soluble salt can dissolve before the solution becomes saturated. For a salt that dissociates into ions, Ksp is the product of the ion concentrations at equilibrium (each raised to its stoichiometric power). When you mix solutions, you compute Q, the ion product, using the actual concentrations present: if Q > Ksp, the solution is supersaturated with respect to that salt, and excess ions will come out of solution as solid to bring Q down to Ksp. If Q < Ksp, dissolution (if solid is present) can continue until Q reaches Ksp; if Q = Ksp, the system is at equilibrium with no net precipitation. Therefore, precipitation occurs when Q exceeds Ksp.