Vapour Pressure Formula

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 using Raoult’s law, which explains how the vapour pressure of a liquid gets changed by the addition of a solute.

             P solution = (X solvent )( Po solvent  )    


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

P solution = the vapour pressure of the solution

solvent = the vapour pressure of the solvent

Solved Examples 

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


Use Raoult’s Law:Psolution = (χsolvent) (P°solvent)

23.80 = (x) (25.756)

x = 0.92405 (this is the solvent mole fraction)

χsolute = 1 – 0.92405

χsolute = 0.07595


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