Ksp Solubility product  Solubility Equilibria
Ksp Solubility product
Lessons
Notes:
In this lesson, we will learn:
 To recall the solubility product expression, K_{sp}.
 How to express solubility and saturation using equilibrium expressions.
Notes:
 In Solubility and ion concentration, we updated our definitions of solubility and saturation. Their correct definitions are about an equilibrium between the dissolved and undissolved state. This means we can write solubility as an equilibrium constant expression.
 For example, a saturated solution of the salt MgCl_{2}, can be described by the equilibrium equation:
MgCl_{2 (s)} $\enspace \rightleftharpoons \enspace$ Mg^{2+}_{(aq)} + 2 Cl^{}_{(aq)}  The solubility product expression for this equilibrium is:
K_{sp} = [Mg^{2+}][Cl^{}]^{2}  The solubility product constant, K_{sp}, is used for any equilibrium constant between dissolved ions and the undissolved compound; it is the K_{eq} for saturated solutions.
 For example, a saturated solution of the salt MgCl_{2}, can be described by the equilibrium equation:
 Remember how K_{a} is an acid strength rating and K_{eq} is basically a product rating in any equilibrium? K_{sp} is effectively a dissolved ions rating! A large K_{sp} value means the concentration of dissolved ions in the saturated solution is large. Therefore the larger the K_{sp} value, the more soluble a substance is.
 Its important to remember the concept of the solubility product constant when dealing with practical problems involving multiple saturated solutions:
 The reason it is called the solubility product CONSTANT is because when the solution is saturated, K_{sp} is equal to the product of the concentration of dissolved ions, regardless of how the solution was prepared or the relative concentrations. The K_{sp} for any given compound at saturation does not change. Recall that:
K_{sp} = [Mg^{2+}][Cl^{}]^{2}
If MgCl_{2} was dissolved in solution until saturation occurred, we would have a stoichiometric ratio of Mg^{2+} and Cl^{} ions, according to the equation:MgCl_{2 (s)} $\enspace \rightleftharpoons \enspace$ Mg^{2+}_{(aq)} + 2Cl^{}_{(aq)}  However, we could make the saturated solution of magnesium chloride another way. Separate compounds may be added in different quantities, such as Mg(NO_{3})_{2} and NaCl which together contribute to the MgCl_{2} K_{sp} equilibrium.
The K_{sp} constant value could be reached if one ion concentration was larger while the other smaller.
Regardless of the proportion of ion concentrations, K_{sp} is constant for a saturated solution and it is the product of the separate dissolved ion concentrations multiplied together.
 The reason it is called the solubility product CONSTANT is because when the solution is saturated, K_{sp} is equal to the product of the concentration of dissolved ions, regardless of how the solution was prepared or the relative concentrations. The K_{sp} for any given compound at saturation does not change. Recall that:

Intro Lesson
What is the solubility product?