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Chemistry LibreTexts

Unit 5: Solubility Equilibrium

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  • Unit 5 Objectives

    By the end of this unit, you will be able to:

    • Define solutions/solutes/solvent/solubility
    • Describe the process by which ionic compounds dissolve in water
    • Solve simple molar solubility problems from Ksp or concentration
    • Describe how the common ion effect affects the solubility of ionic compounds
    • Solve common ion effect and solubility problems
    • Predict precipitation reactions using Q and Ksp 
    • Model complex ion equilibrium
    • Model selective preciptiation experiments 


    • 5.1: Solubility Equilibria and the Solubility Product Constant
      The solubility product (Ksp) is used to calculate equilibrium concentrations of the ions in solution, whereas the ion product (Q) describes concentrations that are not necessarily at equilibrium. The equilibrium constant for a dissolution reaction, called the solubility product (Ksp), is a measure of the solubility of a compound. Whereas solubility is usually expressed in terms of mass of solute per 100 mL of solvent, Ksp is defined in terms of the molar concentrations of the component ions.
    • 5.2: Precipitation
      A mixture of metal ions in a solution can be separated by precipitation with select anions. When a metal ion or a group of metal ions form insoluble salts with a particular anion, they can be separated from others by precipitation. We can also separate the anions by precipitating them with appropriate metal ions.
    • 5.3: Qualitative Chemical Analysis
      In qualitative analysis, the identity, not the amount, of metal ions present in a mixture is determined. The technique consists of selectively precipitating only a few kinds of metal ions at a time under given sets of conditions. Consecutive precipitation steps become progressively less selective until almost all the metal ions are precipitated. Other additional steps are needed to separate metal ions that precipitate together.
    • 5.4: Complex Ion Equilibria
      The formation of complex ions can substantially increase the solubility of sparingly soluble salts if the complex ion has a large Kf. A complex ion is a species formed between a central metal ion and one or more surrounding ligands, molecules or ions that contain at least one lone pair of electrons. Small, highly charged metal ions have the greatest tendency to act as Lewis acids and form complex ions. The equilibrium constant for the formation of the complex ion is the formation constant (Kf).