Chemistry class 12th Relative lowering of vapour pressure

Relative Lowering of Vapor Pressure


1. Introduction

Relative lowering of vapor pressure is one of the colligative properties of solutions. It states that
when a non-volatile solute is added to a solvent, the vapor pressure of the solvent decreases. The
extent of lowering depends only on the number of solute particles, not their identity.


2. Raoult's Law


Raoult's Law states that the partial vapor pressure of each volatile component in a solution is
directly proportional to its mole fraction in the solution.


Mathematically,
P1 = P1^0 * X1


where:
- P1 = Vapor pressure of the solution
- P1^0 = Vapor pressure of the pure solvent
- X1 = Mole fraction of the solvent


3. Formula for Relative Lowering of Vapor Pressure


Relative lowering of vapor pressure is given by:


(P1^0 - P1) / P1^0 = X2


where:
- P1^0 = Vapor pressure of the pure solvent
- P1 = Vapor pressure of the solution
- X2 = Mole fraction of the solute


Since mole fraction of solute is given by:
X2 = n2 / (n1 + n2),