What the common ion effect is
The common ion effect describes how the solubility of a sparingly soluble ionic salt decreases when a solution already contains one of the ions that salt would produce when it dissolves. This happens because of Le Chatelier’s principle: adding more of a product ion shifts the dissolution equilibrium back toward the undissolved solid.
The formula used
For a 1:1 salt MX that dissolves as MX ⇌ M+ + X-, the solubility product is Ksp = [M+][X-]. If the solution already contains a common ion at concentration C0 (for example X- from another soluble salt), and s is the additional molar solubility of MX, then Ksp = s × (s + C0). This calculator solves that equation exactly using the quadratic formula. When no common ion is present, this simplifies to the familiar s = √Ksp.
- Enter the salt’s Ksp value.
- Enter the concentration of the common ion already present in solution (enter 0 for none).
- The calculator compares solubility with and without the common ion.
This calculator applies to simple 1:1 (MX-type) salts and assumes ideal behavior, which is standard for introductory general chemistry problems. It is intended for general chemistry education purposes.