## Raoults Law – Class 12 | Chapter – 2 | Chemistry Short Notes Series PDF for NEET & JEE

Raoults Law: Raoult’s law is a law that describes the relationship between the vapour pressure of a solution and the mole fraction of the solvent in the solution. It states that the vapour pressure of a solution is directly proportional to the mole fraction of the solvent present in the solution.

## Raoults Law

This law is applicable to ideal solutions, which are solutions that follow the rules of Raoults law, and are characterized by the following conditions:

• The intermolecular forces between the solvent and the solute particles are the same as the intermolecular forces between the solvent and the solvent particles.
• The solute particles do not undergo any chemical reaction with the solvent particles.
• The volume of the solute particles is negligible compared to the volume of the solvent particles.

Raoults law can be expressed mathematically as follows:

Psolution = Xsolvent * Psolvent

Where,

• Psolution is the vapour pressure of the solution
• Xsolvent is the mole fraction of the solvent in the solution
• Psolvent is the vapour pressure of the pure solvent

According to Raoults law, when a non-volatile solute is added to a volatile solvent, the vapour pressure of the solution decreases compared to that of the pure solvent, due to the decrease in the mole fraction of the solvent. Therefore, the presence of a solute lowers the vapour pressure of a solution, and the extent of lowering of the vapour pressure is proportional to the mole fraction of the solute.

## Importance of Raoults Law

Raoults law is an important law in the field of chemistry because it provides a simple and accurate method to calculate the vapour pressure of solutions, determine the boiling points of solutions, and estimate the composition of solutions. Here are some of the key importance of Raoults law:

• Predicting vapour pressure: Raoult’s law helps to predict the vapour pressure of a solution based on the mole fraction of the solvent and the vapour pressure of the pure solvent. This is important in many areas of chemistry, including chemical engineering, materials science, and biochemistry.
• Determining boiling points: The boiling point of a solution is influenced by the concentration of solute particles in the solution. Raoult’s law is used to calculate the boiling point elevation of a solution, which is the increase in the boiling point of the solution relative to that of the pure solvent.
• Estimating composition: Raoult’s law can be used to estimate the concentration of solute particles in a solution based on the measured vapour pressure of the solution and the vapour pressure of the pure solvent. This is important in analytical chemistry, where the concentration of solutes in a solution must be accurately determined.
• Understanding intermolecular forces: Raoult’s law assumes that the intermolecular forces between the solvent and solute particles are the same as the intermolecular forces between the solvent particles. When deviations from Raoult’s law occur, this indicates that the intermolecular forces between the solute and solvent particles are different from those between the solvent particles. This provides valuable information about the nature of the intermolecular forces in the solution.

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