Ostwald’s Dilution Law

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Ostwald’s Dilution Law:

  • A mathematical expression of the law of mass actions that gives the relationship between equilibrium constant/dissociation constant, the degree of dissociation and concentration at constant temperature is called as Ostwald’s dilution law.
  • Statement: The degree of ionization (or dissociation)  of any weak electrolyte is inversely proportional to the square root of concentration and directly proportional to the square root of dilution.
  • Explanation: If a is the degree of dissociation of a weak electrolyte, C is its concentration and V is the dilution. Then

Ostwald's Dilution Law 01

  • Ostwald’s dilution law is not applicable to strong electrolytes since their dissociation reaction is irreversible.

 Ostwald’s Dilution Law for Weak Electrolyte:

  • Let one mole of a binary weak electrolyte BA be dissolved in water and the solution is made ‘V’ dm3 by volume. Let ‘α’ be the degree of dissociation of the electrolyte at equilibrium.
  • Weak electrolyte dissociate as

Ostwald's Dilution Law 02

By applying the law of mass action to above equilibrium,

Ostwald's Dilution Law 03



  • The expressions (1) and (2)  are known as Ostwald’s dilution law. Where K = equilibrium constant.
  • For weak electrolyte α is very small, hence 1 – α = 1

Ostwald's Dilution Law 04

  • Thus, the degree of ionization (or dissociation)  of any weak electroyte is inversely proportional to the square root of concentration and directly proportional to the square root of dilution. This relation is known as Ostwald’s law.

Expression for Dissociation Constant of Weak Acid:

  • Let one mole of a binary weak acid HA be dissolved in water and the solution is made ‘V’ dm3 by volume. Let ‘α’ be the degree of dissociation of the acid at equilibrium.
  • Weak acid dissociate in an aqueous solution and equilibrium exists as,

Where Ka = dissociation constant for acid



  • Depending upon the values of C, the degree of dissociation varies in order to keep the value of Ka constant. This is known as Ostwald’s dilution law.

For weak acid  α is very small, hence 1 – α  =  1

This is the expression for dissociation constant of weak acid.

Concentration of H+ ions is given by



Expression for Dissociation Constant of Weak Base:

  •  Let one mole of a weak base BOH be dissolved in water and the solution is made ‘v’ dm3  by volume. Let ‘a’ be the degree of dissociation of the base at equilibrium. Weak base dissociate in an aqueous solution and equilibrium exists as,

Where Kb = Ionisation constant or dissociation constant of base

  • Depending upon the values of C, the degree of dissociation varies in order to keep the value of Kb constant. This is known as Ostwald’s dilution law.

For weak base α is very small, hence 1 – α  =  1



This is the expression for dissociation constant of weak base.

Concentration of OH ions is given by



Science > Chemistry > Ionic EquilibriaYou are Here
Physics Chemistry  Biology  Mathematics

9 Comments

  1. sidra younus

    good explanation

  2. well explained

  3. Nice its very easy to understand

  4. For weak acid or base alpha is very small, hence (1-alpha) term is approx. 1, not alpha.

  5. Festus Kiprotich Maiywa

    Nice Explanation And Easy Following Hence Being Understood Easily

    • Thank you Festus. Our aim is to deliver knowledge in the simplest language so that everybody can understand it. If you like the website spread it through social media in your groups. That is the way we can reach maximum people.

  6. Sourav Debbarma

    First alpha is considered to be very less than 1 then how is alpha equal to 1 at infinite dilution.

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