COLLIGATIVE PROPERTIES OF SOLUTIONS

1
COLLIGATIVE PROPERTIES OF SOLUTIONS
2
Colligative Properties

• Colligative properties depend on the number of
  particles rather than their nature
• Lowering of the vapor pressure (Raoults law)
• Elevation of the boiling point (ebulioscopy)
• Depression of the freezing point (cryoscopy)
• Osmotic pressure

3
Vapor Pressure of Ideal Solution
PAo
PBo
PA PB
A B
Solvent B
Solvent A
Raoults Law PA PAo.xA PB PBo.xB Solvent
pressure lowering (PAo-PA)/PAo xB
4
?Tf and ?Tb
Freezing point depression Kf - cryoscopic
constant or molar depression constant
oC/M Boiling point elevation Kb -
ebulioscopic constant or molar elevation
constant oC/M
5
Osmotic Pressure
vant Hoff equation
6
Molecular Weight Determination
Osmotic Pressure Cryoscopy and Ebulioscopy
7
Non-Ideal Solutions
na nb
na
nb

• ?Emix ? 0, ?Hmix ? 0

8
Non-Ideal Solution Vapor Pressure
Henrys Law PA PAo. ?A xA PB PBo. ?A xB
Attracting components
Repellent components
positive deviation
negative deviation
PAo
PAo
PBo
PBo
1.0
0
0.5
1.0
0
0.5
Mole fraction of B (xB)
Mole fraction of B (xB)
9
Activity and Activity Coefficient

• To account for non-ideality of solutions
• activity is used instead of concentration
• in all equations

Activity ai ?ici
Activity coefficient ?i