Colligative properties

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Colligative properties of a solution

... properties which depend on the osmolarity of a solution (not the type of solutes, just the number of solutes).

Increase in osmolarity:

• Osmotic pressure increase
• Vapour pressure decrease
• Freezing point decrease
• Boiling point increase

NB:

• High osmolarity causes a solution to stay as liquid over larger range of temperatures
• Freezing point is depressed by 1.86C per mole of solvent
• Human plasma (290mOsm/L) freezes at -0.54C

Raoult's law

The reduction of vapour pressure of a solvent is proportional to the molar concentration of the solute.

Azeotrope

An azeotrope is a mixture which vaporizes in the same proportion as the volume concentrations of the components in the solution.
--> i.e. The components will have the same vapour pressure and thus evaporate in the same proportion.

For example,

• Ether and halothane, when ratio is 2:1
• Alcohol and water, when ratio is 96:4
  * As a result, fractional distillation cannot eliminate the last 4% of water from the water/alcohol mix