Chemistry class 12th Colligative properties and molar mass

Colligative Properties and Determination of Molar Mass


1. Colligative Properties

Colligative properties are properties of solutions that depend only on the number of solute particles,
not on their identity. These properties help in determining the molar mass of solutes.


The four main colligative properties are:
a) Relative Lowering of Vapor Pressure
b) Boiling Point Elevation
c) Freezing Point Depression
d) Osmotic Pressure


a) Relative Lowering of Vapor Pressure


When a non-volatile solute is added to a solvent, the vapor pressure of the solvent decreases.
Raoult's Law states:
P1 = P1^0 * X1
where:
- P1 = Vapor pressure of solution
- P1^0 = Vapor pressure of pure solvent
- X1 = Mole fraction of solvent


Relative lowering of vapor pressure is:
(P1^0 - P1) / P1^0 = X2
where X2 is the mole fraction of solute.


b) Boiling Point Elevation


The boiling point of a solution is higher than that of the pure solvent.
Boiling Point Elevation is given by:
Delta Tb = Kb * m
where:
- Delta Tb = Elevation in boiling point