Chapter 15.1 - 15.2

Solubility Product (Ksp) and Related Concepts

Key Concepts

  • Solubility Product (Ksp): A constant that provides a measure of the solubility of a compound in a saturated solution.
  • Molar Solubility: Number of moles of solute per liter of a saturated solution (mol/L).
  • Solubility: Number of grams of solute in 1 liter of a saturated solution.
  • Precipitation Reactions: Occur when two ionic compounds in solution form an insoluble compound (precipitate).

Ksp Expression

  • The Ksp expression for a general ionic compound can be written based on the dissociation in solution. For example,
    • For barium sulfate:
      BaSO<em>4(s)⇌Ba2+(aq)+SO</em>42−(aq)Ksp=[Ba2+][SO42−]BaSO<em>4 (s) \rightleftharpoons Ba^{2+} (aq) + SO</em>4^{2-} (aq) \quad Ksp = [Ba^{2+}][SO_4^{2-}]

Solubility Rules for Ionic Compounds

Generally Soluble Compounds
  • Ions such as Li+, Na+, K+, and NH4+.
  • Nitrates (NO3−) and Acetates (C2H3O2−) are soluble.
  • Halogens (Cl−, Br−, and I−), except when paired with Ag+, Hg2^2+, or Pb2+.
  • Sulfates (SO4^2−) generally soluble, but not with Sr2+, Ba2+, Pb2+, Ag+, or Ca2+.
Generally Insoluble Compounds
  • Hydroxides (OH−) and Sulfides (S^2−) are typically insoluble.
  • Carbonates (CO3^2−) and Phosphates (PO4^3−) are also often insoluble but can dissolve with Li+, Na+, K+, or NH4+.

Solubility Equilibrium Expression

  • Ksp Calculation Example:
    • Ksp=1.07×10−10Ksp = 1.07 \times 10^{-10} for Barium Sulfate (BaSO4), suggesting it is not very soluble.

Precipitation Reactions

  • Q vs. Ksp: Precipitation occurs if the reaction quotient (Q) exceeds Ksp. If Q > Ksp , a precipitate forms.
Steps to Identify Precipitation
  1. Write the formulas of the reactants.
  2. Determine potential products and their solubility.
  3. Calculate Q and compare it with Ksp to predict precipitation.

Practice Problems

  • Example: Ksp of PbCl2 is 1.17 x 10^{-5}. Use stoichiometry to set up an ICE table:
    • Initial, Change, and Equilibrium concentrations to express Ksp:
    • Ksp=[Pb2+][Cl−]2Ksp = [Pb^{2+}][Cl^{-}]^2.

Ice Table Setup

  • Use the ICE table to analyze how much of the solid salt dissolves to determine the ion concentrations at equilibrium.

Example Calculations

  • Given Ksp of a solid, find molar solubility and convert to grams if needed using molar mass:
    1. extMolarsolubility(s)  ⟹  ext{Molar solubility} (s) \implies convert to g/L.
    2. Determine concentration of ions participating in the solubility product expression.

Conclusion

  • Understanding the Ksp, molar solubility, and relation between Q and Ksp is crucial for predicting solubility and precipitation reactions in ionic compounds.