Variables
Description
What is this formula?
This formula calculates the equilibrium concentration of a dissolved ion when a common ion is already present in solution.
When to use it
Use this equation to estimate how the solubility of a sparingly soluble salt changes when one of its ions is supplied by another dissolved compound.
Example
For the salt MX:
MX ⇌ M+ + X−
Given:
Ksp = 1.0×10^-8
Common ion concentration:
[X−] = 0.010 mol/L
Formula:
[M+] = Ksp / [X−]
Substitution:
[M+] = (1.0×10^-8)/(0.010)
Result:
[M+] = 1.0×10^-6 mol/L
The presence of the common ion significantly reduces the dissolved concentration of M+.
Applications
Precipitation control
Analytical chemistry
Water treatment
Chemical separations
Solubility calculations
Note
This formula assumes a 1:1 salt (MX ⇌ M+ + X−) and that the concentration of the added common ion is much larger than the solubility of the salt. Under these conditions, the contribution of the dissolving salt to the common-ion concentration can be neglected. More exact treatments require solving the complete equilibrium expression.