Colligative Properties
How does the solute change the
properties of the solvent?
• Consider aqueous solutions.
• Solvent = water.
• How do the properties of the solution
compare to the properties of pure
water?
Conductivity
• Pure water does not conduct
electricity.
• Some solutions do.
• In order to conduct a current, a
solution must contain ions.
• Conductivity depends on the nature
of the particle – Molecular solutes
do not conduct.
Colligative Properties
• Depend on the number of particles
in solution not on the type of
particles.
– Doesn’t matter if particles are
ions or molecules.
– Concentration of particles does
matter.
What are some colligative
properties?
•
•
•
•
Vapor Pressure Lowering
Freezing Point Depression
Boiling Point Elevation
Osmotic Pressure
• The higher the concentration of solute
particles, the larger the effect!
Colligative Properties
• Useful for determining the nature of
solute after it is dissolved in the
solvent
• Useful for determining the molecular
mass of the solute
Vapor Pressure Lowering
• Presence of nonvolatile solute lowers
the vapor pressure of the solution
compared to that of the pure solvent.
• Has implications for the phase diagram.
• Normal b.p. = temperature at which the
v.p. = 1 atm.
• So, solutions must be heated to a higher
T to reach a v.p. of 1 atm.
• Boiling Point Elevation
Nonvolatile Solute
Pure solvent
Solvent + a
nonvolatile solute
(nrp)
New triple point.
Change in boiling point, Tb
• Tb = Tb – Tb0
• Tb = boiling point of solution – boiling point of
solvent (Tb > Tb0) so Tb is always a positive #.
Tb = Kbmsolute
– Kb = a constant characteristic of the solvent = molal
boiling-point elevation constant (C/m)
– msolute = molality of the solute in solution
Tb = Kbmsolute
• The higher the concentration, the bigger the
effect. So solutes stretch out the
temperature range of the liquid phase.
100C
0C
Pure H2O
More
concentrated
solutions 
Boiling Point of Solution
• What is the boiling point of a solution
containing 478 g of ethylene glycol
(antifreeze) in 3202 g of water?
•
• The molar mass of ethylene glycol is 62
g/mol.
• Kb for water is 0.51Ckg/mol.
• Boiling point of pure water = 100C
Boiling Point of Solution
• Tb = Kbmsolute
• Moles ethylene glycol = 478g / 62 g/mol =7.71
• Molality ethylene glycol = 7.71 mol / 3.202 kg H2O
• = 2.41 m
• Tb = (0.51Ckg/mol)(2.41 mol/kg) = 1.23C
• Boiling Point of Solution = Tb + Tb = 101.23C
Calculating Molar Mass
• msolute = Tb/Kb
• msolute = moles solute/kg solvent
• And moles solute = mass solute/FM
•  (mass solute/FM) = Tb/Kb
kg solvent
Calculating Molar Mass
• Rearranging:
Kb(mass solute)
• FM = (kg solvent)(Tb)
Kb(mass solute)
FM = (kg solvent)(Tb)
• Tb of pure benzene = 80.1C
• Kb for benzene is 2.53 Ckg/mol
• 10.9 grams of unknown solute are
dissolved in 75.8 grams of benzene. Tb
of the solution is 82.1C. Estimate the
molar mass of the solute.
• So Tb = 2.0C
Kb(mass solute)
FM = (kg solvent)(Tb)
• FM = (2.53Ckg/mol)(10.9 g)
(0.0758 kg)(2.0C)
FM = 182 g/mol
Change in freezing point, Tf
• Tf = Tf0 – Tf
• Tf = freezing point of solvent –
freezing point of solution. (Tf0 > Tf)
always positive
Tf = Kfmsolute
– Kf = molal freezing-point depression
constant of the solvent (C/m)
– m = molality of solute in solution
What is the freezing point of a solution
containing 478 g of ethylene glycol in 3202 g of
H2O?
• Formula mass of ethylene glycol = 62 g/mol.
Moles of ethylene glycol = 478 g/62 g/mol =
7.71 moles. For water, Kf = 1.86 C/m.
Molality = 7.71 moles/3.202 kg = 2.41 m.
• Tf = Kfm = (1.86C/m)(2.41 m) = 4.48C.
• Tf = 0.0C – 4.48C = -4.48C
Does it matter if the solute is an
electrolyte or a nonelectrolyte?
• The number of particles will vary.
• Electrolytes will dissolve to produce
positive and negative ions  more
particles per mole of solute.
• For nonelectrolytes, 1-to-1 relationship
between moles of solute and moles of
dissolved particles.
C6H12O6
• Nonelectrolyte
• Dissolves as molecules
• C6H12O6(s)  C6H12O6(aq)
• 1 mole of sugar yields 1 mole of
molecules
NaCl
• Electrolyte (salt)
• Dissolves as ions
• NaCl(s)  Na+(aq) + Cl-(aq)
• 1 mole of salt yields 2 moles of
ions
MgCl2
• Electrolyte
• Dissolves as ions
• MgCl2(s)  Mg2+(aq) + 2Cl-(aq)
• 1 mole of salt yields 3 moles of
ions
Nature of solute
• Colligative properties depend on
concentration of solute particles
• Use experimental data to determine moles
of particles in solution.
Real Life
• Salts do not always exhibit complete
dissociation.
• Result: The actual Tb or Tf is less than
predicted from the dissolving equation.
• Evidence that “Ion-pairing” occurs in solution.
– More ion pairing in concentrated solutions & when
ions have multiple charges.
As a solute is added to a solvent, what
happens to the freezing point & the
boiling point of the solution?
1) The freezing point decreases & the boiling
point decreases.
2) The freezing point decreases & the boiling
point increases.
3) The freezing point increases & the boiling
point decreases.
4) The freezing point increases & the boiling
point increases.
Which solution containing 1 mole of
solute dissolved in 1000 g of water has
the lowest freezing point?
1) C2H5OH(aq)
Highest FP!
2) NaCl(aq)
3) KOH
4) CaCl2
Lowest FP!
Be careful! What
if the question
asked which
solution has the
highest freezing
point?
Of the following solutions, the one that
will freeze at the lowest temperature
contains 1 mole of nonvolatile solute
dissolved in
1)
2)
3)
4)
250 g of solvent
500 g of solvent
750 g of solvent
1000 g of solvent
Which solute, when added to 1000 g of
water, will produce a solution with the
highest boiling point?
1) 29 g of NaCl
2) 58 g of NaCl
About 1 mole of NaCl.
2 moles of ions.
3) 31 g of C2H6O2
4) 62 g of C2H6O2
About 1 mole of C2H6O2.
1 mole of molecules.
Which solution will freeze at the lowest
temperature?
1) 1 g of NaCl dissolved per 100 g of H2O
2) 1 g of NaCl dissolved per 1000 g of H2O
3) 1 g of C12H22O11 dissolved per 100 g of
H 2O
4) 1 g of C12H22O11 dissolved per 1000 g of
H 2O
Which solution will freeze at the lowest
temperature?
1) 1 g of NaCl dissolved per 100 g of H2O
1 g  58.8 g/mole = 0.017 moles  0.034
moles of ions.
3) 1 g of C12H22O11 dissolved per 100 g of
H 2O
1 g  232 g/mole = 0.0043 moles of
molecules
Effect of Nonvolatile Solute
• Boiling Point Elevation
• Freezing Point Depression
• The more particles, the bigger
the effect!
Applications of colligative
properties
• Salting roads in the winter time.
• Using salted ice to make ice cream.
• Antifreeze in radiator to protect water
from overheating or freezing
Osmosis
Osmosis
• A solution and a pure solvent are
separated by a semipermeable
membrane.
• Solvent, but NOT solute molecules, can
pass through the membrane.
Osmosis
• The solvent flows from the region of higher
solvent concentration to the region of lower
solvent concentration.
• Volumesolution  and volumesolvent  as a f(t)
• At equilibrium, the liquid levels stop changing.
• More hydrostatic pressure on the solution
than on the solvent.
Hydrostatic pressure
due to
Another common set-up
Osmotic Pressure
• Minimum hydrostatic pressure that
stops the net flow of solvent across the
membrane into the solution.
• Can be used to characterize solutions
and determine molar mass
• Small concentration of solute 
produces large osmotic pressure
Net movement of solvent into solution over time.
At equilibrium
Note: the phrases
“more and less
concentrated
solution” relate to
the initial and final
conditions of the
solution in the
bulb. The pure
solvent is outside
the bulb.
Net flow of water across
membrane.
At equilibrium
Reverse Osmosis