Colligative properties

Solutions
Colligative Properties
• Changes in colligative properties
depend only on the number of solute
particles present, not on the identity of
the solute particles.
• Among colligative properties are
Vapor pressure lowering
Boiling point elevation
Melting point depression
Osmotic Pressure

Solutions
Vapor Pressure
Because of solute-
solvent intermolecular
attraction, higher
concentrations of
nonvolatile solutes
make it harder for
solvent to escape to
the vapor phase.

Solutions
Vapor Pressure
Therefore, the vapor pressure
of a solution is lower than that
of the pure solvent.

Copyright © by Holt, Rinehart and Winston. All rights reserved.
ResourcesChapter menu
Vapor Pressures of Pure Water and a Water Solution
Chapter 13
Section 2 Colligative Properties of
Solutions

Solutions
Boiling Point Elevation and Freezing
Point Depression
Nonvolatile solute-
solvent interactions
also cause solutions
to have higher boiling
points and lower
freezing points than
the pure solvent.

Solutions
Boiling Point Elevation
The change in boiling
point is proportional to
the molality of the
solution:
Tb = Kb  m
where Kb is the molal
boiling point elevation
constant, a property of
the solvent.Tb is added to the normal
boiling point of the solvent.

Sample Problem E
What is the boiling-point elevation of a solution made from 20.1 g
of a nonelectrolyte solute and 400.0 g of water? The molar mass
of the solute is 62.0 g.
Chapter 13
Solutions
Boiling-Point Elevation, continued

Solutions
Freezing Point Depression
• The change in freezing
point can be found
similarly:
Tf = Kf  m
• Here Kf is the molal
freezing point
depression constant of
the solvent.
Tf is subtracted from the normal
freezing point of the solvent.

Solutions
Boiling Point Elevation and Freezing
Point Depression
Note that in both equations,
T does not depend on what
the solute is, but only on
how many particles are
dissolved.
Tb = Kb  m
Tf = Kf  m

Sample Problem C
What is the freezing-point depression of water in a solution of 17.1 g
of sucrose, C12H22O11, in 200. g of water? What is the actual freezing
point of the solution?
Chapter 13
Solutions
Freezing-Point Depression, continued

Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Chemistry: The Central Science, Eleventh Edition
By Theodore E. Brown, H. Eugene LeMay, Bruce E. Bursten, and Catherine J. Murphy
With contributions from Patrick Woodward
Sample Exercise 13.9 Calculation of Boiling-Point Elevation and
Freezing-Point Lowering
Calculate the freezing point of a solution containing 0.600 kg of
CHCl3 and 42.0 g of eucalyptol (C10H18O), a fragrant substance
found in the leaves of eucalyptus trees.
Practice Exercise

Osmotic Pressure
• A semipermeable membrane allows the passage of some
particles while blocking the passage of others.
• The movement of solvent through a semipermeable
membrane from the side of lower solute concentration to the
side of higher solute concentration is osmosis.
• Osmotic pressure is the external pressure that must be
applied to stop osmosis.
Chapter 13
Solutions

Chapter 13
Solutions
Osmotic Pressure

Osmosis
In osmosis, there is net movement of solvent from the area of
higher solvent concentration (lower solute concentration) to
the area of lower solvent concentration (higher solute
concentration).

Osmotic Pressure
The pressure required to stop osmosis, known as
osmotic pressure, , is
n
V
 = ( )RT = MRT
where M is the molarity of the solution
If the osmotic pressure is the same on both sides of a membrane
(i.e., the concentrations are the same), the solutions are isotonic.

Osmosis in Blood Cells
If the solute
concentration outside
the cell is greater than
that inside the cell, the
solution is hypertonic.
Water will flow out of the
cell, and crenation
results.

Osmosis in Cells
If the solute
concentration outside
the cell is less than
that inside the cell, the
solution is hypotonic.
Water will flow into the
cell, and hemolysis
results.

Osmosis in Cells
Hypotonic
Isotonic
Hypertonic

Solutions
Colligative Properties of Electrolytes
Since these properties depend on the number of
particles dissolved, solutions of electrolytes (which
dissociate in solution) should show greater changes
than those of nonelectrolytes.

Solutions
Colligative Properties of Electrolytes
However, a 1 M solution of NaCl does not show
twice the change in freezing point that a 1 M
solution of methanol does.

Solutions
van’t Hoff Factor
One mole of NaCl in water
gives rise to two moles of
ions. (or very close to two
moles)

Solutions
van’t Hoff Factor
Some Na+ and Cl− re-
associate for a short time, so
the true concentration of
particles is somewhat less
than two times the
concentration of methanol.

Solutions
The van’t Hoff Factor
• Re-association is more likely
at higher concentration.
• Therefore, the number of
particles present is
concentration dependent.

Solutions
We modify the
previous equations
by multiplying by the
van’t Hoff factor, i
Tf = Kf  m  i

Solutions
Determine the expected boiling point of a solution made by
dissolving 25.0g of barium chloride in 0.150kg of water.

Colligative properties

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