# Definition of Vapour Pressure

Whenever the liquid evaporates, the gaseous molecules formed will escape in the air. But, if the liquid is kept in a closed container, the gaseous molecules will not escape in the air but will remain above the liquid. Those evaporated particles will create a pressure above the liquid, which is then known as the vapour pressure.

## What is the Vapour Pressure Formula?

A solution is created when a solid gets dissolved into the liquid. The vapour pressure formed from this solution is lowered by the addition of the solute. Here comes the Vapour pressure formula usingÂ Raoultâ€™s law,Â which explains how the vapour pressure of a liquid gets changed by the addition of a solute.

Â Â Â Â Â Â Â Â Â Â Â Â Â P solution = (X solventÂ )( Po solvent Â )Â  Â Â

X solvent = the mole fraction of the solvent in the solution

P solution = the vapour pressure of the solution

PÂ°solvent = the vapour pressure of the solvent

## Solved ExamplesÂ

Example 1. The vapour pressure of an aqueous solution is found to be 23.80 mmHg at 25 Â°C. What is the mole fraction of solute in this solution? The vapour pressure of water is 25.756 mm Hg at 25 Â°C.

Solution:

Use Raoult’s Law:PsolutionÂ = (Ï‡solvent) (PÂ°solvent)

23.80 = (x) (25.756)

x = 0.92405 (this is the solvent mole fraction)

Ï‡soluteÂ = 1 – 0.92405

Ï‡soluteÂ = 0.07595