• 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.
properties are called
•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.
• 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
𝑠𝑜𝑙𝑣𝑒𝑛𝑡)and the mole fraction of the solvent in the solution 𝑥𝑠𝑜𝑙𝑣𝑒𝑛𝑡 .
𝑃𝑠𝑜𝑙𝑣𝑒𝑛𝑡= 𝑥𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑃○
For a solution containing only one solute,
Where, 𝑥𝑠𝑜𝑙𝑢𝑡𝑒 is the mole fraction of the solute. Manipulating the equation
𝑃𝑠𝑜𝑙𝑣𝑒𝑛𝑡=(1- 𝑥𝑠𝑜𝑙𝑢𝑡𝑒) 𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡- 𝑥𝑠𝑜𝑙𝑢𝑡𝑒 𝑃○
𝑠𝑜𝑙𝑣𝑒𝑛𝑡- 𝑃𝑠𝑜𝑙𝑣𝑒𝑛𝑡=∆P= 𝑥𝑠𝑜𝑙𝑢𝑡𝑒𝑃○
The resulting equation suggests that the decrease in vapor pressure (∆P) is
directly proportional to the solute concentration.