# Solubility Product

Ksp - Solubility Product Constant

The solubility product constant is a simplified equilibrium constant denoted as Ksp which is defined for equilibrium between a solid and its respective ions in a given solution. Its value shows the degree to which a compound can dissociate in water. The greater the solubility product constant, the more soluble is the compound. The Ksp expression for a given salt is the product of the concentrations of the ions. Each concentration is raised to a power which is equal to the coefficient of that ion in a balanced equation to get the solubility equilibrium. The solubility product constants are used for describing the saturated solutions of ionic compounds of relatively low solubility. A saturated solution is in a dynamic equilibrium state between the dissolved and dissociated ionic compound and the undissolved solid. In this article, we will learn about what is the solubility product, what is Ksp, the solubility product definition, and some solubility product numerical.

Solubility Product Definition

The definition of the solubility product is given as follows:

Solubility is defined as the property of a substance known as solute to get dissolved in a solvent for the formation of a solution. The solubility of ionic compounds that disassociate and forms cations and anions in water vary to a great extent. Some compounds are highly soluble and can even absorb moisture from the atmosphere whereas, on the other hand, the others are highly insoluble.

Solubility Product Constant

Now, let us understand what is the solubility product constant in detail.

The solubility product is a type of equilibrium constant whose value depends on temperature. Ksp usually increases with the increase in temperature because of the increased solubility.

Most ionic compounds which are insoluble would still dissolve in water to a small extent. These insoluble compounds are considered to be strong electrolytes since whatever portion of the compound that is dissolved also dissociates. For example, silver chloride dissociates to a small extent in the silver ions and chloride ions when added to water.

AgCl (s)  ↔ Ag⁺ (aq) + Cl⁻ (aq)

This process is written as an equilibrium since the dissociation occurs only to a smaller extent. Hence,  an equilibrium expression is written for the process. You must keep in mind that the solid silver chloride does not have a variable concentration and is therefore not included in the expression. This is the solubility product principle.

Ksp = [Ag⁺] [Cl⁻]

Solubility Product Formula

The solubility product constant is used to describe the saturated solutions of ionic compounds having relatively low solubility.  A saturated solution is said to be in a state of dynamic equilibrium between the ionic compound and the undissolved solid.

The Ksp formula is given in the form of the following equation:

MₓAᵧ (s) → xMy+ (aq) + yAx- (aq)

The general equilibrium constant is written as follows:

Kc = [My+]x [Ax-]y

Significance of the Solubility Product

The solubility depends on a several numbers of parameters which includes the lattice enthalpy of salt and solvation enthalpy of ions in the solution that are of most importance. Let us learn about the significance of the solubility product in detail.

1. Whenever a salt is dissolved in a solvent, the strong forces of attraction of solute that are the lattice enthalpy of its ions need to be overcome by the interactions between the ions and the solvent.

2. The solvation enthalpy of ions is always negative and this means that energy is released during the process.

3. The nature of the solvent finds the amount of energy that is released during the solvation which is solvation enthalpy.

4. Non-polar solvents have a small value of solvation enthalpy which means that this energy is not sufficient for overcoming the lattice enthalpy.

5. Hence the salts are not dissolved in the non-polar solvents. Therefore, for a salt to be dissolved in a solvent, its solvation enthalpy must be greater than its lattice enthalpy.

6. The solubility depends on the temperature and it is varied for every salt.

## Salts are Classified According to their Solubility which is Given in the Table Below:

 Category I Soluble Solubility is > 0.1M Category II Moderately soluble 0.01M < Solubility <0.1M Category III Hardly soluble Solubility is < 0.1M

Solubility Product Numericals

Let us now take a look at some of the Ksp chemistry example.

Example 1:

Determine the solubility product constant for MgF₂.

Solution:

The relevant equilibrium is given by:

MgF₂ (s) ↔ Mg²⁺ (aq) + 2Fe⁻ (aq)

Hence, the associated equilibrium constant is given by:

Ksp = [Mg²⁺] [Fe⁻]²

Example 2:

Give the solubility product constant for Ag₂CrO₄.

Solution:

The relevant equilibrium is given by:

Ag₂CrO₄(s)  ↔ 2Ag⁺ (aq) + CrO$_{4}^{-2}$ (aq)

The associated equilibrium is therefore given by:

Ksp = [Ag⁺ ]² [CrO$_{4}^{-2}$]