# 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 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

By applying the law of mass action to above equilibrium,

• 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

• 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 >You are Here

1. sidra younus

good explanation

well explained

3. Nice its very easy to understand

4. MarkoZ

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

• Hemant More

Thank you for the correction. Correction is made in the article.

5. Festus Kiprotich Maiywa

Nice Explanation And Easy Following Hence Being Understood Easily

• Hemant More

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.

• Hemant More

It is (1 – α) = α and not α = 1

7. vedant

Very nice helped me a lot