17.4: Solubility Equilibria Dissolving & precipitating of salts Solubility rules discussed earlier are generalized qualitative observations of quantitative experiments. Ex: PbCl2 (s) ↔ Pb2+ (aq) + 2 Cl- (aq) Ksp = [Pb2+][Cl-]2 = 1.6 x 10-5 Ksp = solubility-product constant (found in App. D) Recall that both aqueous ions and solid must be present in solution to achieve equilibrium Changes in pH will affect the solubility of salts composed of a weak acid or weak base ion.

Solubility Products Consider the equilibrium that exists in a saturated solution of BaSO4 in water: BaSO4(s) Ba2+(aq) + SO42−(aq) © 2009, Prentice-Hall, Inc.

Solubility Products The equilibrium constant expression for this equilibrium is Ksp = [Ba2+] [SO42−] where the equilibrium constant, Ksp, is called the solubility product. © 2009, Prentice-Hall, Inc.

Solubility Products Ksp is not the same as solubility. Solubility is generally expressed as the mass of solute dissolved in 1 L (g/L) or 100 mL (g/mL) of solution, or in mol/L (M). © 2009, Prentice-Hall, Inc.

Calculating Ksp from solubility 1. Calculate Ksp for Ag2CrO4, if its solubility is 0.022 g/L. (Ans: 6.6 X 10^-5)

Calculating Solubility given Ksp 2. Ksp for MgF2 is 6.4 X 10^-9 at 250C. Calculate its solubility in mol/L and g/L. (Ans: 1.2 X 10^-3 M, 7.3 X 10^-2 g/L)

Factors Affecting Solubility The Common-Ion Effect If one of the ions in a solution equilibrium is already dissolved in the solution, the equilibrium will shift to the left and the solubility of the salt will decrease. BaSO4(s) Ba2+(aq) + SO42−(aq) © 2009, Prentice-Hall, Inc.

Factors Affecting Solubility pH If a substance has a basic anion, it will be more soluble in an acidic solution. Substances with acidic cations are more soluble in basic solutions. © 2009, Prentice-Hall, Inc.

Factors Affecting Solubility Amphoterism Amphoteric metal oxides and hydroxides are soluble in strong acid or base, because they can act either as acids or bases. Examples of such cations are Al3+, Zn2+, and Sn2+. © 2009, Prentice-Hall, Inc.