Colligative Properties
Colligative Properties
Colligative properties depend on quantity of solute
molecules.
Vapor pressure lowering
Boiling point elevation
Freezing point depression
Osmotic pressure
Colligative Properties of Solutions
Definition:
Properties, that depend on the NUMBER of solute
particles present in solution
Concentration:
Osmol per liter or osmolarity
An OSMOL is a mole of solute particle
does not dissociate
C6H12O6
C6H12O6
1 M (6 x 1023)
1 osmol/L
NaCl
1M
does dissociate
Na+ + Cl-
1 osmol/L
2 osmol/L
1 osmol/L
2 x (6 x 1023)
Lowering the Vapor Pressure
Non-volatile solvents
reduce the ability of the
surface solvent
molecules to escape the
liquid.
Therefore, vapor
pressure is lowered.
The amount of vapor
pressure lowering
depends on the amount
of solute.
solvent
solution
Vapor pressure lowering
Raoult’s Law – a
nonvolatile solute will
lower the vapor
pressure of a solvent
Pure water will
have a higher vapor
pressure than salt
water.
Ideal solution: one
that obeys Raoult’s
law.
Raoult’s law breaks
down when the
solvent-solvent and
solute-solute
intermolecular forces
are greater than
solute-solvent
intermolecular forces.

Psolution  Χ solvent Psolvent
Psolution  vaporpressure of thesolution
Χ solvent  molefractionof thesolvent
Χ solvent
nsolvent

nsolvent  nsolute i

Psolvent
 vaporpressure of pure solvent
i  van ' t Hoff factor
Phase Diagram Analysis
Pressure
760 torr
Solid
Liquid
Gas
ΔT
m
Temperature
ΔT
b
Boiling-Point Elevation
Interpret the phase diagram for a solution.
Non-volatile solute lowers the vapor pressure.
At 1 atm (normal bp of pure liquid) there is a lower vapor
pressure of the solution.
 a higher bp is required to reach a vapor pressure of 1
atm for the solution.
ΔTb = Kbmi
Freezing-Point Depression
Solvent
Bp (°C)
Kb (°C m–1)
Mp (°C)
Km (°C m–1)
Water
100
0.51
0
1.86
Acetic acid
118
3.07
17
3.57
Benzene
80
2.53
5.5
5.07
Chloroform
61
3.63
–
–
Camphor
–
–
178
37.7
Cyclohexane
81
2.69
6.5
20.0
ΔTb =
change in bp
Kb = bp constant for solvent
m
= molality of solute
i = van ‘t Hoff factor
Freezing-Point Depression
At 1 atm (normal boiling point of pure liquid) there
is no depression by definition
The solution freezes at a lower temperature (ΔTf)
than the pure solvent.
Decrease in freezing point (ΔTf) is directly
proportional to molality (Kf is the molal freezing-pointdepression constant):
ΔTf = Kfmi
Colligative properties can be used to determine the
MW of an unknown compound.
1. Vapor pressure lowering
Raoult’s law
Psoln = Xsolv P0
Psoln= vapor pressure of solution
Xsolv= mole fraction of solvent
P0= vapor pressure of pure solvent
2. Freezing point depression
DTf = Kf m
DTf= freezing point depression
Kf= cryoscopic constant
m= molality
3. Boiling point elevation
DT= boiling point elevation
DTb = Kb m
Kb= ebullioscopic constant
m= molality
Cryoscopic and ebullioscopic constants
Solvent
Kf
Kb
Water
1.86
0.52
Benzene
5.12
2.53
Phenol
7.40
3.56
°C kg solvent (mol solute) -1
4. Osmotic pressure
p = M  RT
p = osmotic pressure
M = molarity
R = 8.314 JK-1 mol-1
Osmotic Pressure
Semipermeable
membrane: permits passage
of some components of a
solution. Example: cell
membranes and cellophane.
Osmosis: the movement of
a solvent from low solute
concentration to high solute
concentration.
There is movement in both
directions across a
semipermeable membrane.
As solvent moves across
the membrane, the fluid
levels in the arms becomes
uneven.
Eventually the pressure
difference between the
arms stops osmosis.
Dilute solution
Concentrated
solution
Membrane
Osmotic Pressure
Osmotic
pressure, P, is the
pressure required to stop
osmosis.
Isotonic solutions: two
solutions with the same P
separated by a semipermeable
membrane.
Hypotonic solutions: a
solution of lower P than a
hypertonic solution.
Osmosis is spontaneous.
V  nRTi
n
    RTi
V 
 MRTi
Osmotic Pressure & Biology
Red blood cells are surrounded by semipermeable membranes.
Crenation:
red blood cells placed in hypertonic solution (a lower solute concentration
exists in the cell)
osmosis occurs and water passes out of the cell causing the cell to shrivel
up.
Hemolysis is opposite
red blood cells placed in a hypotonic solution (a higher solute concentration
in the cell)
water moves into the cell causing the cell to burst.
To prevent crenation or hemolysis, IV (intravenous) solutions must be
isotonic.
Osmotic Pressure Applications
Pickling food by placing in salty solutions.
Water moves into plants through osmosis.
Salt added to meat or sugar to fruit prevents bacterial infection (a
bacterium placed on the salt will lose water through osmosis and die).
Dialysis machines work by osmosis.
Reverse osmosis is used in desert countries to produce drinking water
from the sea.
P>
P
Salty sea water would
normally draw in
more water to this
side
Drinking water is
produced by
applying a
pressure that
exceeds P.
Electrolytes
‘Anomalous’ behavior:
1. Ability to conduct electric current
2. Greater effect on colligative properties
DTf values: (kf = 1.86°C for H2O)
0.001 m
0.01 m
0.1 m
Sucrose
0.00186
0.0186
0.188
NaCl
0.00366
0.0360
0.348
K2SO4
0.00528
0.0501
0.432
K3[Fe(CN)6]
0.00710
.0626
0.530
van’t Hoff factor: i
i=
measured value
expected value
DTf = Kfm
i=
DTf
Kfm
DTf = i Kfm
DTb = i Kbm
p = i MRT
i values:
0.1
0.01 m
0.001 m
Infinite
dilution
Sucrose
1.01
1.00
1.00
1.00
HCl
1.89
1.94
1.98
2.00
KCl
1.85
1.94
1.98
2,00
MgSO4
1.21
1.53
1.82
2,00
K2SO4
2.32
2.70
2.84
3.00