Ksp – Solubility Product Constant Unit 3 - Equilibrium

Ksp It is a variation on the equilibrium constant for a solute-solution equilibrium. Remember that the solubility equilibrium is based on a saturated solution. Important points to consider are: 1) Some of the solid does dissolve. Not very much, but enough. 2) The substance dissociates upon dissolving. 3) There exists an equilibrium between the undissolved solid and the solvated ions. 4) The higher the Ksp value, the more soluble the compound

Dissociation Equations Our first example is silver chloride, AgCl. When it dissolves, it dissociates like this: AgCl (s)   Ag+ (aq) + Cl- (aq) **Always write the solid as a reactant** The Ksp expression would be then: Ksp = [Ag+] [Cl¯] It turns out that the Ksp value can be either directly measured or calculated from other experimental data. Knowing the Ksp, we can calculate the solubility of the substance and ion concentrations in a very straightforward fashion.

Writing Ksp Expressions Each concentration in the Ksp expression is raised to the power of its coefficient in the balanced equation. In order to write Ksp expressions properly, you must know how ionic substances dissociate in water. That means, you have to know your chemical nomenclature, polyatomic ions, and the charges associated with ion.

Examples Write the Ksp expression for: 1) Sn(OH)2 (s) ↔ Sn2+ (aq) + 2 OH¯ (aq) 2) Solid strontium nitrate in water

Calculating Ksp Given the solubility, calculate the Ksp of the substance. 1).Write the chemical equation for the substance dissolving and dissociating. 2) Write the Ksp expression. 3) Insert the concentration of each ion and multiply out. Review: Calculating Individual Ion Concentrations in Unit 2

Example: Calculate Ksp The concentration of lead ions in a saturated solution of PbI2 at 25˚C is 1.3 x 10-3 M. What is the solubility product constant?

Calculating Ion Concentrations We can also use a known value of Ksp to find out the solubility of a substance. 1) Write out the chemical equation for the substance 2) Write the Ksp expression 3) Solve for x

Example: Calculating Concentrations A) In a 1:1 Ratio The Ksp for MgCO3 at 25˚C is 2.0 x 10-8. What are the ion concentrations in a saturated solution at this temperature?

Example: Calculating Concentrations B) NOT in a 1:1 ratio Calculate the molar concentration for silver chromate in a saturated solution at 25˚C. Ksp = 1.1 x 10-12