Ostwald's Dilution Law describes the relationship between the degree of ionisation of a weak electrolyte and the concentration of its solution. It is a key topic in Class 12 Chemistry (Equilibrium chapter).
Ostwald's Dilution Law was proposed by Friedrich Wilhelm Ostwald in 1888. It applies ONLY to weak electrolytes — strong electrolytes (like HCl, NaOH) are fully dissociated and the law doesn't apply.
Ostwald's Dilution Law: For a weak electrolyte AB dissociating into A⁺ and B⁻:
The degree of dissociation (α) of a weak electrolyte is directly proportional to the square root of the dilution (V) and inversely proportional to the square root of concentration (C).
Consider weak acid HA ⇌ H⁺ + A⁻
Initial: C mol/L, 0, 0 At equilibrium: C(1-α), Cα, Cα
Ka = [H⁺][A⁻] / [HA] = (Cα)(Cα) / C(1-α) = Cα²/(1-α)
For weak electrolytes (α << 1), (1-α) ≈ 1:
Ka ≈ Cα²
Therefore: α = √(Ka/C)
Also: α = √(Ka × V) (where V = 1/C = volume per mole)
No. Strong acids (HCl, H₂SO₄, HNO₃) and strong bases are fully dissociated — α = 1 always. Ostwald's law applies only to weak electrolytes (weak acids like CH₃COOH, weak bases like NH₄OH).
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