Solubility Product and Solubility Equilibria

Definition of the Solubility Product (Ksp)

The solubility product (Ksp) is an equilibrium constant that applies to the dissolution of sparingly soluble ionic compounds in water. It quantifies the extent to which a salt can dissolve to form a saturated solution.

• Solubility product (Ksp): The product of the molar concentrations of the ions each raised to the power of their stoichiometric coefficients in the equilibrium equation.

• For a generic salt MaXb that dissociates as:

• The Ksp expression is:

• Pure solids are not included in the equilibrium expression.

• Example: For AgCl: At 25.0°C,

• A salt with is considered slightly soluble.

• Note: Ksp values can only be directly compared for salts with the same ion ratio.

Relationship Between Ksp and Molar Solubility

Molar solubility (s) is the number of moles of a salt that can dissolve per liter of solution to form a saturated solution. The relationship between Ksp and s depends on the stoichiometry of the salt.

• For a salt that dissociates as :

• For a salt that dissociates as :

• Example: If the molar solubility of AgBr is mol L–1:

• Ksp is dimensionless, as it is a ratio of concentrations.

General Solubility Rules

Patterns of Solubility for Ionic Compounds

Certain patterns help predict whether an ionic compound is soluble or insoluble in water. These rules are useful for laboratory work and understanding precipitation reactions.

The Common Ion Effect

Definition and Impact on Solubility

The common ion effect refers to the decrease in solubility of an ionic compound when a solution already contains one of the ions present in the compound. This is a direct application of Le Châtelier's principle.

• Adding a common ion shifts the dissolution equilibrium to the left, reducing the solubility of the salt.

• Example: Adding Pb(NO3)2 to a saturated solution of PbCl2 increases [Pb2+], causing PbCl2 to precipitate.

• The effect can be quantified using the Ksp expression and a concentration table.

Calculating Solubility in the Presence of a Common Ion

To determine the new solubility, set up an ICE (Initial, Change, Equilibrium) table and use the Ksp expression, making approximations if the change in concentration is small.

• Example: Find the molar solubility (s) of PbI2 in 0.10 M NaI, given at 25°C.

• Since Ksp is small, s is very small, so .

• Set up the Ksp equation:

• Solve for s: M

• The approximation is valid because is much less than 0.10.

Summary Table: Key Concepts

Additional info: The solubility product is a key concept in predicting precipitation, understanding laboratory results, and designing chemical separations. Mastery of Ksp calculations and solubility rules is essential for success in general chemistry.