What is relative lowering of vapour pressure? How is it useful to determine the molar - mass of a solute?

Explanation:

When the solute molecules are non-volatile, then the solution contains only vapour of solvent molecules above it.

When solute is added into it , the vapour pressure of the solution is lower than  pure liquid at a given temperature.

This is known as relative lowering of vapour pressure.

The relative lowering of vapour pressure =   (△p) / (p₁⁰ )=( p₁⁰  -p) / (p₁⁰  ) Here, p₁⁰   is the vapour pressure of pure solvent and Δp is the vapour pressure lowering of solution. To determine the molar-mass of a solute, the following expression is used (△p)/(p₁⁰  )=(W₂ M₁)/(W₁ M₂ ) W_1and W2  are the masses of solvent and solute respectively. M₁  and M₂  are the molar masses of solvent and solute respectively.

Final Answer:When the solute molecules are non-volatile, then the solution contains only vapour of solvent molecules above it.

When solute is added into it , the vapour pressure of the solution is lower than  pure liquid at a given temperature.