What is the common ion effect?

When a soluble salt that shares an ion with a sparingly soluble salt is added to a solution, the equilibrium shifts toward the solid, reducing solubility. For example, adding NaCl to a saturated AgCl solution decreases how much AgCl dissolves because the extra Cl⁻ pushes the equilibrium left.

Does the Ksp change when a common ion is added?

No. Ksp depends only on temperature. What changes is the molar solubility (the amount that actually dissolves). The added ion raises the ion product Q above Ksp momentarily, so some solid precipitates until Q = Ksp again.

How does this calculator solve the equilibrium?

It sets up the exact polynomial Ksp = (m·s)^m · (C₀ + n·s)^n (or the symmetric case) and solves numerically using the Newton-Raphson method. No simplifying assumption that s ≪ C₀ is made, so results are accurate even when C₀ is small.

What stoichiometry types are supported?

AB (e.g. AgCl, BaSO₄), AB₂ (e.g. PbCl₂, CaF₂), A₂B (e.g. Ag₂CrO₄), AB₃ (e.g. AlF₃), and A₂B₃ (e.g. Bi₂S₃). For each type the calculator knows the correct dissolution equation and ICE-table coefficients.

Can I use this for the ion that is NOT in common?

Yes — just select the other ion as the common ion. For example, for CaF₂ in a Ca(NO₃)₂ solution, select the cation (Ca²⁺) as the common ion. The calculator handles both cases.

What does "molar solubility" mean here?

Molar solubility (s) is the number of moles of the sparingly soluble salt that dissolve per liter of solution. In a common-ion solution, s is always smaller than it would be in pure water.

Common Ion Effect Calculator

Find the molar solubility of a sparingly soluble salt when a common ion is already present in solution.

Salt Parameters

Compound Type

Ksp of the salt

Use scientific notation, e.g. 1.77e-10 for AgCl

Common Ion

The cation of the salt is already present in solution.

Initial common ion concentration (mol/L)

Concentration from the added soluble salt before any dissolution

Enter the salt parameters and click Calculate.

Molar Solubility in Common-Ion Solution

Pure Water

Suppression Factor

ICE Table

Step

Step-by-Step Solution

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Summary

The common ion effect suppresses the solubility of a sparingly soluble salt when a soluble source of one of its ions is already present in solution. This calculator solves the equilibrium expression exactly — no simplifying approximations — given the Ksp, the compound stoichiometry, which ion is in common, and the initial concentration of that ion. It covers AB, AB₂, A₂B, AB₃, and A₂B₃ compound types and shows a full ICE-table step-by-step breakdown.

How it works

Enter the Ksp of the sparingly soluble salt (e.g. 1.77e-10 for AgCl).
Select the compound stoichiometry (AB, AB₂, A₂B, AB₃, or A₂B₃).
Choose which ion is the common ion — cation or anion.
Enter the initial concentration of the common ion from the added soluble salt.
Click Calculate to get the molar solubility, equilibrium concentrations, and an ICE-table breakdown.

Use cases

Predict how much AgCl dissolves in NaCl or AgNO₃ solution.
Determine the solubility of CaF₂ in a NaF solution for an analytical experiment.
Verify that a precipitation reaction goes to completion when excess common ion is added.
Solve general and analytical chemistry homework on ionic equilibria.
Estimate the residual ion concentration after adding a precipitating reagent.
Compare solubility in pure water versus common-ion solution to quantify the suppression.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu