Colligative
Properties

Colligative
Properties
• Some properties of solutions
depend on the nature of
their constituent substances,
while others depend solely
on the number of the solute
particles in the solution.
These quantity-dependent
properties are called
colligative properties.

Solutions that depend
on the nature of their
components can be
classified as
electrolytic or non-
electrolytic.

Electrolytes
•Are particles that ionize in a solution; as a
result, they conduct electricity.

Non-electrolytes
•Are those that do not ionize in a solution
and does not conduct electricity.

•Since colligative properties depend on the
number of solute particles in solution,
electrolytes have a greater effect on the
lowering of vapor pressure and freezing
point, increase in the boiling point, and on
osmotic pressure. In predicting the behavior
of colligative properties, the solute particles
are assumed to have negligible interactions.

Vapor Pressure Lowering
• The vapor pressure lowering of a solution with a nonvolatile (can
easily evaporate) solute is always lower than that of the pure solvent.
The relationship between the vapor pressure of the solution and the
vapor pressure of the solvent depends on the concentration of the
solute in the solution.

Raoult’s Law
• States that the vapor pressure of the solvent over a solution (𝑃𝑠𝑜𝑙𝑣𝑒𝑛𝑡) is
equal to the product of the vapor pressure of the pure solvent
(𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡)and the mole fraction of the solvent in the solution 𝑥𝑠𝑜𝑙𝑣𝑒𝑛𝑡 .
In expression:
𝑃𝑠𝑜𝑙𝑣𝑒𝑛𝑡= 𝑥𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡
For a solution containing only one solute,
𝑥𝑠𝑜𝑙𝑣𝑒𝑛𝑡=1- 𝑥𝑠𝑜𝑙𝑢𝑡𝑒
Where, 𝑥𝑠𝑜𝑙𝑢𝑡𝑒 is the mole fraction of the solute. Manipulating the equation
gives
𝑃𝑠𝑜𝑙𝑣𝑒𝑛𝑡=(1- 𝑥𝑠𝑜𝑙𝑢𝑡𝑒) 𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡
= 𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡- 𝑥𝑠𝑜𝑙𝑢𝑡𝑒 𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡
𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡- 𝑃𝑠𝑜𝑙𝑣𝑒𝑛𝑡=∆P= 𝑥𝑠𝑜𝑙𝑢𝑡𝑒𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡
The resulting equation suggests that the decrease in vapor pressure (∆P) is
directly proportional to the solute concentration.

Sample problem:
• Calculate the vapor pressure of a solution containing 600g of sugar
(𝐶12 𝐻22 𝑂11) and 1000 g of water at 25°C. The 𝑃○