3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.4. Physiology
                  3.1.4.8. General physiology
                      3.1.4.8.1. Osmolality and tonicity
 3.1.4.8.1.1. Colligative properties 

Colligative properties

[KB2:??? page]

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