 # Vapor 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 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 which explains how the vapour pressure of a liquid gets changed by the addition of a solute.

P solution = X solvent  x   Po solvent

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

P solution = the vapor pressure of the solution

solvent = the vapor pressure of the solvent

Examples of vapour pressure formula

Question 1.What will be the vapor pressure of a solution at 25°C containing 3.5 moles of glucose in 10.0 moles of water?

Answer: It is important to calculate the mole fraction of water in order to solve out Raoult’s law.

XH2O=0.741;

Use mole fraction of water to calculate for Raoult’s law.

P solution = X solvent  x   Po solvent

P solution =0.741 x 23.8 torr

P solution =17.6 torr

Question 2. What will be the vapor pressure of a solution at 25°C when 25.5 grams of glucose (C6H12O6) is dissolved in 212 grams of water?

Answer: It is important to convert both the grams of glucose and grams of water into moles in order to solve out mole fraction of water.

Now solve for the mole fraction of water.

H2O=0.988;

Finally, calculate the partial pressure of the solution.

P solution = X solvent  x   Po solvent

P solution=0.988 x 23.8 torr

P solution=23.5 torr