colligativeproperties-ganapathi-200429073946.ppt

Dr. M. Ganapathi
Assistant Professor of Chemistry
Department of Chemsitry
Vivekananda College
Tiruvedakam West
ganchemistry@gmail.com

 Colligative property
 A property of a solvent that depends on the total
number of solute particles present
 There are four colligative properties to consider:
 Vapor pressure lowering (Raoult’s Law)
 Freezing point depression
 Boiling point elevation
 Osmotic pressure
Colligative Properties

Colligative Properties – Vapor Pressure
 A solvent in a closed container reaches a
state of dynamic equilibrium.
 The pressure exerted by the vapor in the
headspace is referred to as the vapor
pressure of the solvent.
 The addition of any nonvolatile solute (one
with no measurable vapor pressure) to any
solvent reduces the vapor pressure of the
solvent.

 Nonvolatile solutes reduce the ability of the surface
solvent molecules to escape the liquid.
 Vapor pressure is reduced.
 The extent of vapor pressure lowering depends on the
amount of solute.
 Raoult’s Law quantifies the amount of vapor
pressure lowering that is observed.

 Raoult’s Law:
PA = XAPO
A
where PA = partial pressure of the solvent (A)
vapor above the solution (ie with
the solute)
XA = mole fraction of the solvent (A)
Po
A = vapor pressure of the pure
solvent (A)

Example: The vapor pressure of pure water at 110o
C is
1070 torr. A solution of ethylene glycol and water has a
vapor pressure of 1.10 atm at the same temperature.
What is the mole fraction of ethylene glycol in the
solution?
Both ethylene glycol and water are liquids. How do you
know which one is the solvent and which one is the
solute?

 Ideal solutions are those that obey Raoult’s Law.
 Real solutions show approximately ideal behavior
when:
 The solution concentration is low
 The solute and solvent have similarly sized molecules
 The solute and solvent have similar types of
intermolecular forces.

 Raoult’s Law breaks down when solvent-solvent and
solute-solute intermolecular forces of attraction are
much stronger or weaker than solute-solvent
intermolecular forces.

Colligative Properties – BP Elevation
 The addition of a nonvolatile solute
causes solutions to have higher boiling
points than the pure solvent.
 Vapor pressure decreases with addition
of non-volatile solute.
 Higher temperature is needed in order
for vapor pressure to equal 1 atm.

Colligative Properties- BP Elevation
 The change in boiling point is proportional to the
number of solute particles present and can be related to
the molality of the solution:
DTb = Kb
.
m
where DTb = boiling point elevation
Kb = molal boiling point elevation constant
m = molality of solution
The value of Kb depends only on the identity of the
solvent (see Table 13.4).

Colligative Properties - BP Elevation
Example: Calculate the boiling point of an aqueous
solution that contains 20.0 mass % ethylene glycol
(C2H6O2, a nonvolatile liquid).
Solute =
Solvent =
Kb (solvent) =
Tb = Kb  m

Molality of solute:
Tb =
BP = 102.1o
C

Example: The boiling point of an aqueous solution that
is 1.0 m in NaCl is 101.02o
C whereas the boiling point of an
aqueous solution that is 1.0 m in glucose (C6H12O6) is
100.51o
C. Explain why.

Example: A solution containing 4.5 g of glycerol, a
nonvolatile nonelectrolyte, in 100.0 g of ethanol has a
boiling point of 79.0o
C. If the normal BP of ethanol is
78.4o
C, calculate the molar mass of glycerol.
Given: DTb =
mass solute =
mass solvent =
Kb = 1.22o
C/m (Table 13.4)
Find: molar mass (g/mol) Tb = Kb  m

Step 1: Calculate molality of solution
Step 2: Calculate moles of solute present
Step 3: Calculate molar mass

Colligative Properties - Freezing Pt Depression
 The addition of a nonvolatile
solute causes solutions to
have lower freezing points
than the pure solvent.
 Solid-liquid equilibrium line
rises ~ vertically from the
triple point, which is lower
than that of pure solvent.
 Freezing point of the solution
is lower than that of the pure
solvent.

 The magnitude of the freezing point depression is
proportional to the number of solute particles and can
be related to the molality of the solution.
Tf = Kf  m
where DTf = freezing point depression
Kf = molal freezing point depression constant
m = molality of solution
The value of Kf depends only on the identity of the
solvent (see Table 13.4).

Example: Calculate the freezing point depression of a
solution that contains 5.15 g of benzene (C6H6) dissolved in
50.0 g of CCl4.
Given: mass solute =
mass solvent =
Kf solvent =
Find: DTf
Tf = Kf  m

Molality of solution:
Tf =

Example: Which of the following will give the lowest
freezing point when added to 1 kg of water: 1 mol of
Co(C2H3O2)2, 2 mol KCl, or 3 mol of ethylene glycol
(C2H6O2)? Explain why.

 Reminder: You should be able to do the following as
well:
 Calculate the freezing point of any solution given
enough information to calculate the molality of the
solution and the value of Kf
 Calculate the molar mass of a solution given the value
of Kf and the freezing point depression (or the
freezing points of the solution and the pure solvent).

Colligative Properties - Osmosis
 Some substances form semipermeable membranes, allowing
some smaller particles to pass through, but blocking other
larger particles.
 In biological systems, most semipermeable membranes allow
water to pass through, but solutes are not free to do so.
 If two solutions with identical concentration (isotonic
solutions) are separated by a semipermeable membrane, no
net movement of solvent occurs.

 Osmosis: the net movement of a solvent through a semipermeable
membrane toward the solution with greater solute concentration.
 In osmosis, there is net movement of solvent from the area of lower
solute concentration to the area of higher solute concentration.
 Movement of solvent from high solvent concentration to low
solvent concentration

 Osmosis plays an important role in living systems:
 Membranes of red blood cells are semipermeable.
 Placing a red blood cell in a hypertonic solution (solute
concentration outside the cell is greater than inside the cell)
causes water to flow out of the cell in a process called
crenation.

Colligative Properties
 Placing a red blood cell in a hypotonic solution (solute
concentration outside the cell is less than that inside
the cell) causes water to flow into the cell.
 The cell ruptures in a process called hemolysis.

 Other everyday examples of osmosis:
 A cucumber placed in brine solution loses water and
becomes a pickle.
 A limp carrot placed in water becomes firm because
water enters by osmosis.
 Eating large quantities of salty food causes retention
of water and swelling of tissues (edema).

colligativeproperties-ganapathi-200429073946.ppt

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