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Lesson
Colligative Properties
Key Words
colligative properties:
molal boiling point
elevation constant:
molal freezing point
depression constant:
osmosis:
osmntic pressure:
properties that depend upon the number of solu
te
particles in solution and not the properties of the
solute
amount a one-molal solution of nonelectrolyte will
raise the boiling point of a solution
amount a one-molal solution of nonelectrolyte will
lower the freezing point of a solution
movement of solvent molecules from an area
of high
solvent concentration to an area of low solvent
concentration
pressure that must be applied to a solution to prev
ent
osmosis
KEY IDEAS
L
Freezing point, boiling point, and osmotic press
ure are alt colligative piopertles
(kuh-UG-uh-Uv). These properties depend t.qon
the number of solute paitictes Wi solution
rather than the properties of the solute itself.
The lowering of the freezing point of
water is a colilgative property that aplal
ns how antifreeze works in car engines.
Osmotic pressure Is a coffigative property that
needs to be understood by heafth-care
workers wto administer intravenous (IV) solutio
ns.
Boiling Point Elevation. The boiling poin
t of a solution depends on the
concentration of solute particles. As the
concentration of the solution
increases, its boiling point also increases. For
example, a one-molal (1 m)
solution of sugar in water boils at 100.52°C
. A two-molal (2 m) solution of
sugar water boils at 101.04°C. The molal boili
ng point elevation constant
(ehl-uh-vay-shuhn KAHN-stuhnt) for water
is 0.52°C per molal solution.
1. What Is the boding point of a 3 m sugu solu
tion?
When most molecular compounds disso
lve, they separate into molecules.
These compounds an nonelectrolytes. Sugar
is an example of a nonelectrolyte.
When ionic compounds dissolve, they sepa
rate into ions. These compounds
are electrolytes. MaCi is an example of an
electrolyte. When it dissolves,
each NaCl unit separates into two ions. Thus
, one mole of NaCI will produce
twice as many dissolved partides as one mole
of sugar s a result the boiling
pon nf i cne-molal Cl n’ lutinn of aCt
is ele ated twIce as much as 0 52°C
.L* A
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2. What Is the boiling point of 2 m NaCJ?
Freezing Point Depression. Freezing points of solutions also depend on
the concentration of solute partides. The molal freezing point depression
constant (dee-PRESH-uhn KAHN-stuhnt) for water is 1.86°C per molal
solution. A I m solution of sugar in water freezes at -1.86°C. A 2 m solution
of sugar water freezes at —3J2°C.
3. What Is the freezing point of a 3 m suga solution?
Calculating with K. To find the boiling point or freezing point of any
solution, use this equation:
T K 7flfl
Here T is the temperature change from the normal boiling or freezing
temperature. The elevation or depression constant is K. The molality of the
solution is m. The number of particles per formula unit is n. For example,
2 is the formula for calcium chloride. YOU can tell how many solute
CaC1
2
particles it forms by the number of ions in its formula. There are one Ca
unit.
formula
the
for
ion and twoCl ions, which is a total of three particles
2 solution? Since
Sample Problem: What is the boiling point of a 1.5 m CaCI
2 separates into three ions, n =3.
CaC1
T Km n
T (0 52°C/m)(1 5 m)(3)
T=234°C
Boiling point = 100°C + 2.34°C = 102.34°C
In this expression, the temperature change (2.34°C) is added to the normal
boiling point of water (100°C). The result is the boiling point of the solution.
Osmosis. Membranes are thin covering sheets of material, such as the cell
membranes of the cells of the human body and other living things. These
membranes allow some molecules, but not others, to flow through them.
The movement of solvent molecules, usually water, through a membrane is
osmosis (ahs-MOH-sihs). Osmosis takes place across cell membranes.
Fig. 32-1
S
ESugar War-.
solut
soIutiontQr
ornhrnp
4
%
Membrane
Membrane
(a)
(b)
CL)
Ltian 32
CoiUv Popqrs
Part a of Fig. 32-1 shows a membrane
that separates a sugar solution and
water. The solvent—water—flows back
and forth across the membrane.
However, the rate of the flow of water is
greater toward the solution with
the higher concentration of sugar. This
is because water molecules tend
to move from an area of high water concen
tration to an area of low water
concentration. As the flow of solvent contin
ues, the liquid levels become
uneven, as shown in Part 1’ of the diagram.
This difference in level causes a
downward force known as osmotic pressu
re (ahs-MAHT-ihk PRESH-uhr).
If the same amounts of pressure were
applied to the solute side of the tube,
osmosis would stop. The number of
solute particles determines osmotic
pressure. A high-particle concentration
results in a high osmotic pressure, as
shown in Partc of Fig. 32-1.
TAKE
ANOTHER
Boiling points and freezing points of liquids
are affected by the number of
dissolved particles per formula unit. The chart
below compares water solutions
of some substances, their boiling points,
and their freezing points.
Solute (aq)
LOOK
KCI
CaCl
2
NaC
O
3
H
2
Al(N0
)
3
4
CuSO
12
C
11
HO
Number oI Particles
per Formula Unit
Boiling point
Freezing point
C)
2
101.04
-3.72
3
2
4
101.56
11>1.04
-5.58
-3.72
2
102.08
101.04
I
-3.72
10(152
-1.86
-7.44
If you put a cucumber in a saltwater
solution, osmosis takes place, as shown
in Fig. 32-2. The cucumber loses water
by osmosis. Water molecu. s flow
in both directions across the cucumber’s
skin, but more water flow: out of
the cucumber than into it. This turns
the cucumber into a pickle.
Fig. 32-2
water solution
Water
C ttcumber
Urit 6
So1utn
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Check Your
Understanding
Fill in the blanks with the correct answers.
4 water solution’
4. What is the boiling point of 2 in ZnSO
5. What is the freezing point of 6
in
2 water solution?________
HO
1
C
11
6. The movement of a solvent thmugh a membrane is called
7.
solution.
The flow of solvent is always toward the
8. The force needed to stop the flow of molecules across a membrane is
called
Circle the letter of the correct answer.
9. What is the freezing point of a solution that contains one mole of
2 in 1000 g of water?
Mg[3r
a. 0.00°C
b. -1.86°C
c. -3.72°C.
d. -5.58°C
10. Which one-molal water solution will have the highest boiling point?
a. NaC1
4
b. MgSO
2
c. Cal
2
0
1
H
C
d. 6
ii. As the molalily of a solution increases, its osmotic pressure
C. remains the same.
a. increases. b. decreases.
12. As the molality of a solution increases, its freezing point
c. remains the same.
a. increases. b. decreases.
Answer the following in complete sentences.
13. Why is the boiling point elevation of a 1 in solution of NaCI twice as
much as that of a 1
in
solution of sugar?
14. Describe the flow of water molecules across a membrane between pure
water and a solution of sugar.
Lesson 32 Co*pe Proptt*kl