How to express solubility and saturation using equilibrium expressions.
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 MgCl2, can be described by the equilibrium equation:
MgCl2 (s)⇌ Mg2+(aq) + 2 Cl-(aq)
The solubility product expression for this equilibrium is:
Ksp = [Mg2+][Cl-]2
The solubility product constant, Ksp, is used for any equilibrium constant between dissolved ions and the undissolved compound; it is the Keq for saturated solutions.
Remember how Ka is an “acid strength rating” and Keq is basically a “product rating” in any equilibrium? Ksp is effectively a “dissolved ions rating”! A large Ksp value means the concentration of dissolved ions in the saturated solution is large. Therefore the larger the Ksp value, the more soluble a substance is.
It’s 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, Ksp will be equal to the PRODUCT OF the concentration of dissolved ions, regardless of how the solution was prepared or the relative concentrations. The Ksp for any given compound at saturation does not change. Recall that:
Ksp = [Mg2+][Cl-]2
If MgCl2 was dissolved in solution until saturation occurred, we would have a stoichiometric ratio of Mg2+ and Cl- ions, according to the equation:
MgCl2 (s)⇌ Mg2+(aq) + 2Cl-(aq)
However, we could make the saturated solution of magnesium chloride another way, for example two separate compounds may be added in different quantities, such as Mg(NO3)2 and NaCl, or other compounds, or varying quantities.
The Ksp constant value could be reached if one ion concentration was larger while the other smaller.
Regardless of the proportional ion concentrations, at saturation Ksp is constant and is the product of the separate dissolved ion concentrations multiplied together.
What is the solubility product?
Ksp Solubility product
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