molar solubility

Meaning: In a saturated solution, undissolved solid is in equilibrium with its ions. If s mol/L of the salt dissolves, each ion concentration equals s times its subscript in the formula. For AgCl, [Ag⁺] = [Cl⁻] = s. For PbCl₂, [Pb²⁺] = s and [Cl⁻] = 2s. The solid is left out of the Ksp expression.

Equation: For AB, Ksp = s². For AB₂, Ksp = (s)(2s)² = 4s³. Here s is the molar solubility in mol/L (1 mol/L = 1000 mol/m³) and Ksp is the solubility product, written without units on the MCAT.

On the MCAT: You may be given Ksp and asked for s, or given s and asked for Ksp. Passages often add a common ion, such as AgCl in 0.10 M NaCl. Then [Cl⁻] is about 0.10 M, so s = Ksp/0.10, and s falls compared with pure water. Adding acid raises the solubility of salts with basic anions, such as hydroxides, fluorides and carbonates. Comparing the ion product Q with Ksp predicts whether a precipitate forms. Q > Ksp means precipitation.

Common confusion: Ksp and molar solubility are not the same quantity. A larger Ksp does not always mean a larger s when the salts have different formulas. AgCl has s = √Ksp, while Ag₂CrO₄ has s = ∛(Ksp/4). Also, solubility in g/L needs the molar mass: multiply s by it.