Common Ion Effect

How shared ions affect equilibrium and solubility

Principle

Adding an ion already present in equilibrium decreases solubility of a salt

Applies Le Chatelier's Principle: equilibrium shifts to consume added ion

AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq)

K_sp = [Ag⁺][Cl⁻] = 1.8 × 10⁻¹⁰

Scenario 1: AgCl in pure water

Let s = solubility → [Ag⁺] = [Cl⁻] = s

s² = 1.8 × 10⁻¹⁰ → s = 1.3 × 10⁻⁵ M

Scenario 2: AgCl in 0.1 M NaCl solution

[Cl⁻] ≈ 0.1 M (common ion from NaCl)

[Ag⁺] × 0.1 = 1.8 × 10⁻¹⁰ → [Ag⁺] = 1.8 × 10⁻⁹ M

Result: Solubility decreases from 1.3 × 10⁻⁵ M to 1.8 × 10⁻⁹ M

~10,000 times less soluble!

Common ion must be from strong electrolyte (fully dissociated)
Effect is most dramatic when common ion concentration is much larger than solubility
K_sp value doesn't change; only solubility decreases
Equilibrium shifts left (toward solid) when common ion added

Calculate Ksp and solubility

Calculate K from concentrations

Ksp equilibrium expression

Equilibrium shifts and stress

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