Common Ion Effect Calculator

Calculate solubility reduction from common ion effect. Find molar solubility of a sparingly soluble salt in a solution already containing one of its ions.

💧 Equilibrium📐 Ksp = [M][X]^x with [X] = common ion conc + s🧪 Chemistry
Ksp of salt
Concentration of common ion (mol/L)
Stoichiometry of common ion (1 or 2)
Stoichiometry of other ion
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Common Ion Effect CalculatorSee formulaKsp = [M][X]^x with [X] = common ion conc + smol/L

Step-by-Step Examples

Example 1
AgCl in 0.100 M NaCl

Ksp(AgCl)=1.8x10^-10. Common ion: Cl^- at 0.100 M.

  • Ksp = [Ag+][Cl^-] = [Ag+](0.100 + s) ~ [Ag+] x 0.100
  • s = [Ag+] = Ksp/[Cl^-] = 1.8x10^-10/0.100
  • s = 1.8x10^-9 M
  • Pure water: s = sqrt(1.8x10^-10) = 1.34x10^-5 M
✓ Solubility drops from 1.34x10^-5 to 1.8x10^-9 M (7400x reduction)
Example 2
BaSO4 in 0.010 M Na2SO4

Ksp(BaSO4)=1.1x10^-10. Common ion: SO4^2- at 0.010 M.

  • s = Ksp/[SO4^2-] = 1.1x10^-10/0.010 = 1.1x10^-8 M
  • Pure: s = sqrt(1.1x10^-10) = 1.05x10^-5 M
✓ Solubility: 1.1x10^-8 M (1000x less than pure water)
Example 3
CaF2 in 0.050 M NaF

Ksp(CaF2)=3.9x10^-11. Common ion: F^- at 0.050 M.

  • Ksp=[Ca^2+][F^-]^2 = s x (0.050)^2 (since 0.050 >> 2s)
  • s = 3.9x10^-11/(0.050)^2 = 1.56x10^-8 M
✓ s = 1.56x10^-8 M (much less than pure water solubility)

Real-World Applications

💧
Analytical Gravimetry
Adding excess precipitating agent exploits common ion effect to ensure complete precipitation.
🌊
Water Chemistry
Presence of Ca^2+ from other salts reduces solubility of CaCO3 in groundwater.
🧪
Drug Formulation
Common ions can dramatically reduce drug salt solubility; must account for in formulation.
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Buffer Design
Common ion effect on weak acid/base equilibria is exploited in buffer preparation.

Common Mistakes to Avoid

⚠️
Assume common ion dominates

If [common ion] >> s, then [common ion] + s ~ [common ion]. This simplifies the calculation greatly.

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Check assumption validity

After solving for s, verify that s << [common ion]. If not, use the quadratic formula.

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Common ion must match formula

Only ions that appear in the Ksp expression cause the common ion effect. Na+ in NaCl does not affect Ksp of AgCl.

Frequently Asked Questions

What is the common ion effect? ▾
Adding an ion common to the Ksp expression suppresses dissolution. AgCl solubility in 0.1 M NaCl is 7000x less than in pure water.
Why does common ion reduce solubility? ▾
Le Chatelier: adding Cl- shifts AgCl dissolution equilibrium left. More AgCl stays solid, less dissolves.
How is common ion used in gravimetric analysis? ▾
Add excess precipitating reagent (e.g., excess Cl- for AgCl). Common ion drives precipitation to completion, leaving <0.01% of Ag+ in solution.
Does the common ion effect apply to acids/bases? ▾
Yes. Adding CH3COO- (from NaOAc) to acetic acid suppresses ionization: common ion H+ and CH3COO-. This is how buffers work.
What is salting out? ▾
Adding a neutral electrolyte (not a common ion) to a salt solution can also reduce solubility by activity effects. Different from common ion effect.

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💧 Solubility Product💧 Solubility Calculator⚖️ Le Chatelier⚖️ Equilibrium Constant🔬 pH Calculator

Formula Explorer connections

Interpretation: This relationship connects hydrogen-ion activity, dissociation, conjugate ratios or titration stoichiometry to acid–base behavior. Assumption: Concentration approximates activity mainly in dilute solutions. Temperature, ionic strength, acid strength and equilibrium approximations affect accuracy.

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