Colligative properties

Chemistry

1. Dr. M. Ganapathi
Assistant Professor of Chemistry
Department of Chemsitry
Vivekananda College
Tiruvedakam West
[email protected]

2. 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

3. 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
pressureof the solvent.
The addition of any nonvolatile solute (one
with no measurable vapor pressure) to any
solvent reduces the vapor pressure of the
solvent.

4. Colligative Properties –Vapor Pressure
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 Lawquantifies the amount of vapor
pressure lowering that is observed.

5. Colligative Properties –Vapor Pressure
Raoult’s Law:
P
A= X
AP
O
A
where P
A= partial pressure of the solvent(A)
vapor above the solution (ie with
the solute)
X
A= mole fraction of the solvent(A)
P
o
A= vapor pressure of the pure
solvent(A)

6. Colligative Properties –Vapor Pressure
Example:The vapor pressure of pure water at 110
o
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?

7. Colligative Properties –Vapor Pressure
Ideal solutionsare 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.

8. Colligative Properties –Vapor Pressure
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.

9. Colligative Properties –BP Elevation
The addition of a
nonvolatile solute causes
solutions to have higher
boiling pointsthan 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.

10. Colligative Properties-BP Elevation
The change in boiling point is proportional to the
number of solute particles presentand can be related
to the molality of the solution:
DT
b= K
b
.
m
whereDT
b= boiling point elevation
K
b= molal boiling point elevation
constant
m = molality of solution
The value ofK
b depends only on the identity of the
solvent(see Table 13.4).

11. Colligative Properties -BP Elevation
Example:Calculate the boiling point of an aqueous
solution that contains 20.0 mass % ethylene glycol
(C
2H
6O
2, a nonvolatile liquid).
Solute =
Solvent =
K
b(solvent) =
T
b= K
bm

12. Colligative Properties -BP Elevation
Molality of solute:
T
b=
BP = 102.1
o
C

13. Colligative Properties -BP Elevation
Example:The boiling point of an aqueous solution that
is 1.0 m in NaCl is 101.02
o
C whereas the boiling point of an
aqueous solution that is 1.0 m in glucose (C
6H
12O
6) is
100.51
o
C. Explain why.

14. Colligative Properties -BP Elevation
Example: A solution containing 4.5 g of glycerol, a
nonvolatile nonelectrolyte, in 100.0 g of ethanol has a
boiling point of 79.0
o
C. If the normal BP of ethanol is
78.4
o
C, calculate the molar mass of glycerol.
Given: DT
b=
mass solute =
mass solvent =
K
b= 1.22
o
C/m (Table 13.4)
Find:molar mass (g/mol) T
b= K
bm

15. Colligative Properties -BP Elevation
Step 1: Calculate molality of solution
Step 2: Calculate moles of solute present
Step 3: Calculate molar mass

16. 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.

17. Colligative Properties -Freezing Pt Depression
The magnitude of the freezing point depression is
proportional to the number of solute particlesand can
be related to the molality of the solution.
T
f= K
fm
whereDT
f= freezing point depression
K
f= molal freezing point depression
constant
m = molality of solution
The value ofK
f depends only on the identity of the
solvent(see Table 13.4).

18. Colligative Properties -Freezing Pt Depression
Example:Calculate the freezing point depression of a
solution that contains 5.15 g of benzene (C
6H
6) dissolved
in 50.0 g of CCl
4.
Given: mass solute =
mass solvent =
K
fsolvent =
Find:DT
f
T
f= K
fm

19. Colligative Properties -Freezing Pt Depression
Molality of solution:
T
f=

20. Colligative Properties -Freezing Pt Depression
Example:Which of the following will give the lowest
freezing point when added to 1 kg of water: 1 mol of
Co(C
2H
3O
2)
2, 2 mol KCl, or 3 mol of ethylene glycol
(C
2H
6O
2)? Explain why.

21. Colligative Properties -Freezing Pt Depression
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 K
f
Calculate the molar mass of a solution given the
value of K
fand the freezing point depression (or the
freezing points of the solution and the pure solvent).

22. 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.

23. Colligative Properties -Osmosis
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 lowersoluteconcentrationto
the area of highersoluteconcentration.
Movement of solvent from high solvent
concentration to low solventconcentration

24. Colligative Properties -Osmosis
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 greaterthan inside the cell)
causes water to flow out of the cell in a process called
crenation.

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

26. Colligative Properties -Osmosis
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).