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Problem
 Approximately
Colligative Properties I
0.0043 g of
oxygen can be dissolved in
100 mL of water at 20 OC
 Express this concentration in
terms of parts per million
 Assume the density of water
is 1g /1 mL
Solution
Solution
 Known:
 mass
of O2 = 0.0043 g
 volume of H2O = 100 mL
 ppm
=?
Colligative Properties
physical properties of a
solution differ from those of the
pure solvent used to make the
solution
 For example, pure H2O freezes at
0 OC and boils at 100 OC
 The
Colligative Properties
aqueous solution of ethylene
glycol allows the antifreeze in
your car’s cooling system to
freeze below 0 OC and to boil
above 100 OC!
 An
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Colligative Properties
 The physical properties of the
solvent are changed by the
addition of the solute
 What is interesting is that this
difference has nothing to do with
the identity of the solute, only
how much of it there is dissolved
in the solvent!
Colligative Properties
A
property that depends only
upon the number of particles, and
not upon their identity, is called a
colligative property
Colligative Properties
 The
4 colligative properties are:
Vapor Pressure Lowering
point elevation
 Freezing point depression
 Vapor pressure lowering
 Phenomenon of osmotic pressure
that a liquid in a closed
container will establish an
equilibrium with its vapor
 The pressure generated by the
vapor is call the vapor pressure of
the substance
Vapor Pressure Lowering
Vapor Pressure Lowering
 Boiling
 Adding
a solute to a solvent
always lowers the vapor pressure
of the solvent
 Why?
 There are less solvent particles to
escape to the vapor phase!
 Recall
 As
a result, a mixture of
solute + solvent has a lower vapor
pressure than the pure solvent
does by itself
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Vapor Pressure Lowering
Vapor Pressure Lowering
 The
more solute is dissolved in the
solvent, the lower the vapor pressure
of the solvent at any given
temperature
 Ionic solutes have a greater effect
upon the vapor pressure of a solvent
than molecular solutes do
 Why?
Vapor Pressure Lowering
Boiling Point Elevation
 Since the addition of a solute to a
solvent will lower the solvent’s
vapor pressure, it follows that it
will take more heat energy to
make the solvent boil (P = 1 atm)
 Therefore we see that a solution
will have a higher boiling point
than that of the pure solvent!
Boiling Point Elevation
Freezing Point Depression
increase in the boiling point
is directly proportional to the
number of solute particles present
in the solution
 The
a solution doesn’t consist of
pure solvent, not all of the total
solution particles will freeze at the
solvent’s normal freezing point
 The temperature will need to be
lowered below the normal freezing
point to make the solution freeze
 Since
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Phase Diagrams
Osmotic Pressure
two solutions of identical osmotic
pressure are separated by a
semipermeable membrane, no
osmosis will occur
 The two solutions are said to be
isotonic
 If one solution is of lower osmotic
pressure, it is hypotonic with respect
to the higher concentration solution
 Therefore
 If
Osmotic Pressure
 If one solution is of higher
osmotic pressure, it is hypertonic
with respect to the lower
concentration solution
 This effect is responsible for why
the electrolyte and protein
concentration of our blood is so
important
Osmotic Pressure
 Remember:
 The solvent follows the solute
we see that a solution
will have a lower freezing point
than that of the pure solvent!
Osmotic Pressure
Osmotic Pressure
 Imbalances
in the solute
concentration in the blood →
imbalances in the solute
concentration of the extracellular
fluid → fluid flow in/out of cells
 This is the process that occurs during
heart or renal failure that leads to the
formation of edema (excess
extravascular fluid)
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