Question

State and explain Raoult’s law.

Answer 1

Statement of Raoult’s law : The law states that, at constant temperature, the partial vapour pressure of any volatile component of a solution is equal to the product of vapour pressure of the pure component and the mole fraction of that component in the solution.

Let P₀ and P be the respective vapour pressures of a pure volatile component and a solution. If x is the mole fraction of a solvent then by Raoult’s law,

P = x × P₀.

Explanation : Consider a solution containing two volatile components A and B having mole fractions x₁ and x₂ respectively.

Let \(P_1^0\)and \(P_2^0\)be the vapour pressures of pure components (or liquids) A and B respectively.

Then by Raoult’s law, vapour pressure of component A = P₁ = X₁ x \(P_1^0\), vapour pressure of component B = P₂ = X₂ x \(P_2^0\).

Here P₁ and P₂ represent partial vapour pressures of the two liquid components in the solution.

Hence the total vapour pressure, P_T of the solution will be,

With the help of above equation, the vapour pressures of solutions having different concentrations (or mole fractions) can be calculated.