1. Colligative Properties in Solution
Colligative properties - physical properties that depend on the number but not the kind of solute particles
4 important colligative properties for solutions:
1) Vapour pressure depression
2) Boiling point elevation
3) Freezing point depression
4) Osmotic pressure

2. Vapour Pressure Depression
The addition of a non-volatile solute (i.e. sugar) to a volatile solvent lowers the vapour pressure of that system.
Raoult’s Law PSolvent = XSolventP oSolvent
where, PSolvent = vapour pressure of the solvent in the solution
XSolvent = mole fraction of solvent in the solution
P oSolvent = vapour pressure of pure solvent
PSolvent
P oSolvent

3. Vapour Pressure Depression
The total vapour pressure may decrease or increase if two volatile liquids (A & B) are mixed together. PTotal = PA + PB
Raoult’s Law is still obeyed because the partial pressure of A decreases with the addition of B and the partial pressure of B decreases with the addition of A. PA = XAPAo PB = XBPBo
P = PA + PB
PAo
PBo PB PA

4. Deviations from ideal behavior
Positive deviation is due to the association
between solute-solute and solvent-solvent
(differences in polarity between the solute
and the solvent)
Negative deviation is due to the
association between solute-solvent
(strong intermolecular interactions
between solute and solvent molecules)

5. Boiling Point Elevation
The boiling point of a solvent increases with the addition of a non-volatile solute.
Boiling point - The point at which the vapour pressure of a liquid/solution is equal to the external pressure.
Raoult’s law says that the vapour pressure of a solvent will decrease with the addition of a non-volatile solute therefore, the boiling point of the solution must be higher than the pure solvent.
Tb = Kbm
Tb = Tb - Tbo
Kb = molal boiling point constant
(Kb depends on the solvent, oC/m)
m = molality of the solute
(mol solute/kg solvent)

6. Freezing Point Depression
Tf = Kfm
Tf = Tfo - Tf
Kf = molal freezing point constant
(Kf depends on the solvent, oC/m)
m = molality of the solute
(mol solute/kg solvent)

7. Van’t Hoff Factor, i
colligative properties depend on the number of particles and not their identity therefore, we need to take into account the dissociation of electrolytes
van’t hoff factor, i - a measure of the extent of dissociation of an electrolyte
electrolyte ideal i Actual i
1.00m m
Sucrose
NaCl
K2CO
K3[Fe(CN)6]

8. Osmotic Pressure
Osmotic Pressure - the pressure necessary to stop osmosis
Osmosis - the spontaneous flow of a solvent from an area of low concentration of solute to an area of high concentration of solute.
 = MRT (dilute solution)
if 1.00 g/ml then
 = mRT
for a non-electrolyte
 = ? for a strong electrolyte
 = iMRT

9. For a 1.25m sucrose solution: vapour pressure depression = 0.524 torr
boiling point elevation = 0.640oC
freezing point depression = 2.32oC
osmotic pressure = 28.6 atm (2174 torr)
Osmotic pressure is used to find the Molecular Weight of small quantities of sparingly soluble proteins.