Colligative Properties

• Depend on the concentration of solute particles, but not on chemical identity.

• In the case of a solute that does not ionize, “concentration of solute particles” has the same meaning as “solute concentration”.

Some Colligative Properties

• Vapor pressure lowering– Boiling point elevation– Freezing point depression

• Osmotic pressure

Why Vapor Pressure Lowering?

• Results when a non-volatile solute is dissolved in a volatile solvent.

• Only some of the surface molecules have the ability to vaporize, compared to all of the surface molecules in the case of a pure solvent.

• As a result, the vapor pressure of the solution is less than that of a pure solvent.

Molecular-Level Explanation for Vapor Pressure Lowering (image 1 of 3)

Molecular-Level Explanation for Vapor Pressure Lowering (image 2 of 3)

Molecular-Level Explanation for Vapor Pressure Lowering (image 3 of 3)

An Interesting Illustration of the Difference in Vapor Pressure Between Solvent and Solution

Raoult’s Law

• P = X . Po

• Obeyed exactly, only for ideal solutions• For non-ideal solutions, Raoult’s law is more

closely followed for dilute solutions

Raoult’s Law Applied to Both Solvent and Solute (when both are volatile)• P = X . Po

• P = X . Po

• P = P + P = X . Po + X . Po = (1-X) . Po + X . Po

Vapor Pressure Lowering Changes the Boiling and Freezing Points

Equations for Boiling Point Elevation and Freezing Point Depression

• DTb = Tb – Tob and DTb = Kb . cm

• DTf = Tf – Tof and DTf = -Kf . cm