ColHgafive Properties:
Freezing-Point
Depression and
Molar Mass
EXPERIMENT
18
OBJECTIVE
To become familiar with colligative properties and to use them to determine
the molar mass of a substance.
ring and ring stand
clamp
wire gauze
thermometer
large test tube
wire stirrer
Bunsen burner and hose
600-mL beaker
APPARATUS
AND CHEMICALS
sulfur, "roll" or precipitated; or
unknown solid (2 g)
naphthalene (50 g)
2-hole robber stopper with slit
towel
wide-mouth glass bottle
weighing paper
WORK IN PAIRS BUT EVALUATE YOUR DATA INDEPENDENTLY.
DISCUSSION
The major component is called the solvent, and the minor component is
called the solute. Since the sQlution is primarily composed of solvent, physical properties of a solution resemble those of the solvent. Some of these
physical properties, called colligative ptvperties, am independent of the uature
of the solute and depend only upon the solute concentration. The colligative
properties include vapor-pressure lowering, boiling-point elevation, freezing-point lowering, and osmotic pressure. The vapor pressure is just the
escaping tendency of the solvent molecules. When the vapor pressure of a
solvent is equal to atmospheric pressure, the solvent boils. At this temperature the gaseous and liquid states of the solvent are in dynamic equilibrium,
and the rate of molecules going from the liquid to the gaseous state is equal
to the rate of molecules going from the gaseous state to the liquid state. It has
been found experimentally that the dissolution of a nonvolatile solute (one
with very low vapor pressure) in a solvent lowers the vapor pressure of the
solvent, which in turn raises the boiling point and lowers the freezing point.
This is shown graphically by the phase diagram given in Figure 18.1
You are probably familiar with some common uses of these effects: Antifreeze is used to lower the freezing point and raise the boiling point of
coolant (water) in an automobile radiator; and salt is used to melt ice. These
effects are expressed quantitatively by the colligative-property law, which
states that the freezing point and boiling point of a solution differ Dom those
of the pure solvent by amounts that are directly proportional to the molal
189
19{}
Experiment 18 - Colligative Properties: Freezing-Point Depression and Molar Mass
,Freezing point of solution
~
, Freezing point of solvent
~
b.p. elevation
f.p. lowering
Temperature~ °C ~
,~ FIGU RE 18.1 Phase diagram for a solvent and a solution,
concentration of the solute. This relationship is expressed by Equation [1] for
the freezing-point lowering and boiling-point elevation:
~T =Km
[~]
where AT is the freezir~g-point lowering or boiling-point elevation. K is a
constant that is specific for each solvent, and m is the molality of the solution
(number of moles solute peril000 g solvent). Some representative constants,
boiling points, and freezing points are given in Table 18.1. For naphthalene,
the solvent used in this experiment, the molal freezing-point depression constant (K~p) has a value of 6.9°C/m,
EXAMPLE 18.1
What would be the freezing point {~f a solution containing 19.5 g of biphenyl
(CI2Hm) dissolved in I00 g of naphthalene if the normal freezing point of naphthalene is 80.6~C?
18,1 Molal Freezing-Point and Boiling-Point Constants
Solvent
Freezing
point (°C)
CH3COOH (acetic acid)
16.6
CbH6 (benzene)
5,zt
-63.5
CHCI~ (chloroform)
C~HsOH (ethyl alcohol) -114.I
0,0
H~O (water)
80.6
Ci~~ (naphthalene)
C~H~z (cyclohexane)
6.6
Ktp(°C/m)
Boiling
point{°C)
3.90
5.12
4.68
-1,86
6,9
20,4
118.1
80.2
61.3
78.4
100.0
218
80,7
Kl}p(°C/ra)
2.93
2.53
3.63
1.22
2.79
Laboratory Experiments
SOLUTION:
Moles C12H10
19.5 g
0.127 tool
154 g/mo!
Moles C12H10 /0.127 mo!~
1000 g naphthalene = ~ ~ ](1~000 g)
= 1.27 m
AT = (6.9°C/m)(1.27 m)
= 8.76°C or 8.8°C
Since the freezing point is lowered, the observed freezing point of this solution
will be
80.6oc - 8.8oc = 71.8oc
Since the molal freezing-point-depression constant is known, it is possible to
obtain the molar mass of a solute by measuring the freezing point of a solution and the weight of both the solute and solvent.
EXAMPLE 18.2
What is the molar mass of urea if the freezing point of a solution containing 15 g
of urea in 100 g of naphthalene is 63.3°C?
SOLUTION: The freezing point of pure naphthalene is 80.6°C. Therefore, the
freezing-point lowering (AT) is:
&T - 80.6°C - 63.3°C - 17.3°C
From Equation [1] above,
17.3°C - Kfpln
=- 17.3°C
m
We know
that K~p for naphthalene is 6.9°C/m. Therefore, the molality of this
solution is
17.3°C
m = -- - 2.5 m
6.9°C/m
Remember that molality is the number of moles of solute per 1000 g of solvent.
In our solution there are 15 g urea !n 100 g of naphthalene, or 150 g of urea in
1000 g of naphthalene. Thus
150 g = 2.5 mol
1 mol = 60 g
The molar mass of urea is, therefore, 60 g/mol
In this experiment you will determine the molar mass of either sulfur or an
unknown. You will do this by determining the freezing-point depression of
a naphthalene solution having a known concentration of either sulfur or
your unknown. The freezing temperature is difficult to ascertain by direct
visual observation because of a phenomenon called supercooling and also
because solidification of solutions usually occurs over a broad temperature
range. Temperature-time graphs, called cooling curves, reveal freezing temperatures rather clearly. Therefore, you will study the rate at which liquid
191
192 Experiment !8 ° Colligafive Properties: Freezing-Point Depression and Molar Mass
Liquid
is freezing
Solid is cooling
Freezing
Cooling curve for
pure solvent
solution
begins
Cooling curve for solution
Cooling curves for a solvent and a solution,
naphthalene and its solutions cool and will construct a cooling curve similar
to the one shown in Figure 18.2.
You will construct cooling curves for both the pure solvent and the solution. Figure 18.2 shows how the freezing point of a solution must be determined by extrapolation of the cooling curve. Extrapolation is necessary
because as the solution freezes the solid that is formed is ~ssentially pure
solvent and the remaining solution becomes more and more concentrated.
Thus its freezing point lowers continuously. Clearly, supercooling produces
an ambiguity in the freezing point and should be minimized. Stirring the
solution helps to minimize supercooling.
PROCEDURE ,j. A. Cooling Curve for Pure Naphthalene
Q~ Weigh a large test tube to the nearest 0 01 g. Add about 15 g of naphthalene
and weigh again. The difference in weight is the weight of naphthalene,
Assemble the apparatus as shown in Figure 18.3; be certain to use a split
two-hole rubber stopper. Carefully insert the thermometer into the hole that
has been slit. Bend the stirrer so that the loop encircles the thermometer.
~ Fill your 600-mL beaker nearly full of water and heat it to about 85°C,
Clamp the test tube in the water bath as shown in Figure 18,3. When most o~
the naphthalene has melted, insert the stopper containing the thermometer
and stirrer into the test tube; make certain that the thermometer is not resting on the bottom of or touching the sides of the test tube, When all of the
naphthalene has melted, stop heating, remove the beaker of water, and dry
the outside of the test tube with a cloth towel, Place the test tube in a widemouthed bottle that contains a piece of crumpled paper in the bottom to
lessen the chance that impact of the test tube with the bottle will cause the
bottle to break. The purpose of the wide-mouth bottle is to minimize drafts.
(~ Record temperature readings every 30 s while you are stirring. When the
freezing point is reached, crystals will start to form, and the ~emperature
Laboratory Experiments 193
Wire stirrer
Support thermora~ with split
two-hole rubber stopper
Test %ube
600-mL beaker
ater ba~h
~phthaleneintesttube
FIGURE 18.3 Apparatus for determination of cooling /urve.
will remain constant. Shortly after this/the naphthalene will solidify to the
point where you can no longer stir it. t,, "~_.q::~,~ ~,
~,,~ ~- _,~ --~ ~
Your lab ins[rttctor will direct you to perform either~~,r procedure C~)
B. Determination of the Molar Mass of Sulfur
Using weighing paper, weigh to the nearest 0.01 g about 1.2 to ~.5 g of
sulfur. CLEAN UP ANY SULFUR SPILLS IN THE BALANCE. Replace the
test tube in the water bath and heat until all the naphthalene has melted.
Gently remove the stopper, making sure that no naphthalene is lost, and add
the sulfur to the test tube. Replace the stopper and stir gdntly until all the
sulfur has dissolved. Remove the water bath, dry the test tube with a towel,
and insert the test tube in a wide-mouth glass bottle containing a crumpled
p~ece of paper. Record the temp, erature every 30 s until all tl~ naphthalene
Cleanup To clean out the test tube at the end of the experiment, heat the
test tube in a water bath until the naphthalene just melts. Care should be taken
not to heat the thermolneter beyond its teraperature range. Be careful, becattse ~aphthalene is flammable. Remove the stopper and pour the molten naphthalene
on a crumpEed wad of paper. When the naphthalene has solidified, throw
both the paper and solid naphthalene into a waste receptacle. DO NOT
POUR LIQUID NAPHTHALENE INTO THE SINK!
~-~
194
Experiment I8 ¯ Colligative Properties: Freezlng-Point Depression and Molar Mass
C. Determination of the Molar Mass of an Unknown*
Place the test tube in the water bath and heat until all the naphthalene has
melted. Using weighing pape~’,~weigh about 2 g of your unknown to the
nearest 0.0I g. Gently remove the stopper from the test tube, making sure
that no naphthalene is lost, and add your unknown to the test tube. Replace
the stopper and stir gently until alI the unknown has dissolved. Remove the
water bath, dry the test tube with a towel, and insert the test tube in a widemouth glass bottle containing a crumpled piece of paper. Record the temperature every 30 s until all the naphthalene has solidified. Clean up as
described in Part B above.
REVIEW
QUESTIONS
Before beginning this experiment in the laboratory, you should be able to
answer the following questions:
1.
Distinguish between soh¢te and solvent.
2. List three colligative properties and suggest a rationale for the choice
of the word colligative to describe these properties.
3. Distinguish between volatile and nonvolatile substances.
4. What effect does the presence of a nonvolatile solute have upon (a)
the vapor pressure of a solution, (b) the freezing point, and (c) the boiling point?
5. What is the molality of a solution that contains 1,5 g urea (molar mass
= 60 amu) in 200 g of benzene?
6. What is supercooling? How can it be minimized?
7. Calculate the freezing point of a solution containing 6.50 g of benzene
in 160 g of chloroform.
8. A solution contaLning 1.00 g of an unknown substance in 12.5 g of
naphthalene was found to freeze at 75.4°C. What is the molar mass of
the unknown substance?
How many grams of NaNO3 would you add to 250 g of HaO in order
to prepare a solution that is 0,200 molal in NaNO3?
Define molality and molority.
*Instructor Note: cyclohexane may be substituted for naphthalene in this part of this experiment, but it must be cooled with an ice bath and kept away from flames.
.,
Desk
Name
Da~e
Laboratory Instructor
REPORT SHEET
EXPERIMENT
Colligative Properties: 18
Freezing-Point Depression
and Molar Mass
1. Weight of test tube + naphthalene
2. Weight of test tube
3. Weight of naphthalene
g
g
g
4. Weight of paper + sulfur or unknown
5. Weight of paper
6. Weight of sulfur or unknown
g
g
g
Cooling-curve data
Pure naphthalene
Naphthalene + sulfur or unknown
7. Freezing point of pure naphthalene, from cooling curve
8. Freezing point of solution of sulfur or unknown in naphthalene
9. Molality of sulfur or unknown (show calculations)
195
196 Report Sheet ° Colligative Properties: Freezing-Point Depression and Molar Mass
10. Molar mass of sulfur or unknown (show calculations) ~
HAND IN YOUR COOLING CURVES WITH YOUR REPORT SHEET.
QUESTIONS
1. What are the major sources of error in this experiment?
2. Suppose your thermometer consistently read a temperature 1.2° lower than the correct temperature
throughout the experiment. How would this have affected the molar mass you found?
3. If the freezing point of the solution had been incorrectly read 0.3° lower than the true freezing point, would
the calculated molar mass of the solute be too high or too low? Explain your answer.
4. Arrange the following aqueous solutions in order of increasing freezing points (lowest to highest temperature): 0.10 m glucose, 0.10 m BaC12, 0.20 m NaC1, and 0.20 rn Na2SO4.
5. What mass of NaCI is dissolved in 150 g of water in a 0.050 m solution?
Laboratory Experiments 197
6. Calculate the molalities of some commercial reagents from the following data:
HC1
36.465
Molar mass (amu)
Density of solution (g/mL) 1.19
37.2
Weight %
Molarity
12.I
HC2H302
60.05
1_~)5
99.8
17.4
NH3(aq)
17.03
0.90
28.0
14.8
7. A solution of 2.00 g of para-dichlorobenzene (a clothes moth repellant) in 50.0 g of cyclohexane freezes at
1.05°C. What is the molar mass of this substance?
COOLING CURVE FOR PURE NAPHTHALENE
~+
COOLING CURVE FOR SOLUTION OF SULFUR OR UNKNOWN IN NAPHTHALENE