Ion Product

\[Q=[M^{m+}]^a[X^{x-}]^b\]

Variables

Qion product

Mcation concentration (mol/L)

Xanion concentration (mol/L)

astoichiometric coefficient of the cation

bstoichiometric coefficient of the anion

Description

What is this formula?

The ion product is the product of the ion concentrations raised to their stoichiometric coefficients at any moment, whether or not equilibrium has been reached.

When to use it

Use this formula to determine whether a solution is unsaturated, saturated, or supersaturated with respect to a sparingly soluble salt.

Example

For the salt MX2:

MX2 ⇌ M2+ + 2X−

Given:

[M2+] = 2.0×10^-5 mol/L

[X−] = 1.0×10^-3 mol/L

Formula:

Q = [M][X]^2

Substitution:

Q = (2.0×10^-5)(1.0×10^-3)^2

Result:

Q = 2.0×10^-11

If Ksp = 5.0×10^-11, then:

Q < Ksp

The solution is unsaturated and additional salt can dissolve.

Applications

Precipitation prediction

Solubility analysis

Water chemistry

Analytical chemistry

Chemical process control

Note

The ion product Q has the same mathematical form as the solubility product Ksp, but they are not the same quantity. Q can be calculated at any instant using the current ion concentrations, whereas Ksp is an equilibrium constant.

Interpretation:

Q < Ksp → Unsaturated solution

Q = Ksp → Saturated solution

Q > Ksp → Supersaturated solution and precipitation is thermodynamically favored.