Solubility Product (Ksp) – Definition, Applications

Ksp refers to the solubility product constant. It is an application of the law of mass action to find the solubility of a sparingly soluble salt.

When a slightly soluble ionic compound is added to water, some of it dissolves and the remaining settles at the bottom. Afterward, dynamic equilibrium is established between this undissolved solid and its ions in the saturated solution.

Ksp is defined as the product of the equilibrium concentration of ions, each raised to a power that is its coefficient in the balanced chemical equation.

In other words, Ksp is the product of molar solubilities of ions at equilibrium.

Ksp is dependent on temperature. It is normally a small quantity at room temperature. 

Moreover, Ksp value can be determined from the solubility of a compound, and vice versa.

The solubility of an ionic compound generally increases by increasing the temperature. In fact, this is the case for ionic compounds whose enthalpy of solution is endothermic.

Generally, Ksp expression of a sparingly soluble ionic compound AaBb can be written as

Ksp = [A+b]a [B-a]b

For the general equation,

AaBb(s) ⇌ aA+b(aq) + bB-a(aq)

This shows that the solubility product constant, Ksp, is equal to the product of the concentration of ions each having an exponent equal to the number of its ions in the formula unit of the compound.

An example would be to add CaF2 to water. The following equilibrium will be established:

CaF2(s) ⇌ Ca2+(aq) + 2F(aq)

Kc expression for this equilibrium will be written as

Kc = [Ca+2] [F]2 / [CaF2]

As CaF2 is sparingly soluble, its concentration almost remains constant.

Kc [CaF2] = [Ca2+] [F]2

So we get the following Ksp expression

Ksp = [Ca2+] [F]2

Precipitation Reaction

A reaction that takes place when two or more solutions, when mixed, yield an insoluble solid precipitate, is known as a precipitation reaction.

Ion product (Q’) is obtained by using given concentrations of ions rather than the equilibrium concentrations in the Ksp expression. For CaF2, it is written as

Q’ = [Given concentration of Ca+2] [Given concentration of F]2

Comparing Ksp value with Q’ can help us to predict whether precipitation will occur or not:

If Ksp > Q’, precipitation will not occur, and the solution is unsaturated.

If Ksp < Q’, the solution is momentarily supersaturated. Therefore, precipitation will occur until the concentration of ions reaches that of equilibrium.

If Ksp = Q’, the solution is saturated with the ions but there will be no precipitation.

Concepts Berg

What is the Solubility Product, Ksp?

The solubility product constant, Ksp, is the product of concentration of ions at equilibrium, where each concentration of ion has an exponent equal to that of their coefficient in the balanced chemical equation.

What is Ksp and Q?

Solubility product, Ksp is the product of concentration of ions in a saturated solution (at equilibrium). On the other hand, ion product quotient, Q, is the product of concentration of ions in a supersaturated, saturated, or unsaturated solution.

If Q > Ksp, supersaturated solution and precipitation occurs until Q = Ksp.

If Q < Ksp, unsaturated solution and more solid can be dissolved.

If Q = Ksp, saturated solution but no precipitation (system at equilibrium).

What is meant by solubility product?

It is the product of concentration of ions at equilibrium with its solid ionic compound in a saturated solution.

What does low Ksp mean?

Low Ksp for an ionic compound indicates that the compound is less soluble. On the contrary, higher the value of solubility product constant, Ksp, more soluble the compound will be.

How do you write Ksp equations?

For the general equation,

AmBn(s) ⇌ mA+n(aq) + nB-m(aq)

Ksp expression can be written as,

Ksp = [A+n]m [B-m]n

As an example, in the balanced equation, MgF2(s) ⇌ Mg2+(aq) + 2F(aq), the coefficient of Mg2+ is 1 whereas the coefficient of F is 2. These values will be put as their respective exponents. So the Ksp expression will be written as

Ksp = [Mg2+] [F]2

Why is understanding Ksp important?

Ksp gives a general idea of how soluble an ionic compound is. A higher Ksp value corresponds to higher solubility.

In addition, it can be compared with ion product quotient to predict whether precipitation will occur or not.

How do you calculate Ksp from saturated solution?

From a saturated solution, we can find out the molar solubilities of each ionic specie. In order to calculate the Ksp, we take the product of the molar solubilities, each raised to an exponent that corresponds to its coefficient in the balanced chemical equation.

How does Ksp relate to solubility?

Ksp is directly proportional to solubility. A higher Ksp value corresponds to higher solubility.

Is Ksp the solubility product constant for a saturated solution?

Yes, Ksp is the equilibrium constant. However, an equilibrium can only be established when the solution is saturated.

How do you find solubility given Ksp and molarity?

If Ksp of an ionic compound is provided, the solubility of its component ions can be found by taking the relevant root of the Ksp value.


Was this article helpful?