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COLLIGATIVE PROPERTIES:
DETERMINING THE MOLCULAR WEIGHT OF
A COMPOUND BY FREEZING POINT
DEPRESSION CONSTANT
Objectives: Determine the freezing point of pure cyclohexane, and the freezing point of
a mixture of cyclohexane and an organic compound.
Use the data obtained to determine the molecular weight of the organic compound
In this experiment, you will:
 Determine the freezing point of a pure solvent, cyclohexane;
 Determine the freezing point of a mixture of cyclohexane and benzophenone;
 Determine the molecular weight of benzophenone.
A colligative property is a
physical property which
depends only on the
number of particles (think
moles!) in the system, not
what the particles are
(That is, ideally, 1 mole of
Hydrogen atoms will
have the same colligative
properties as 1 mole of
Hemoglobin molecules.
◦
BEFORE YOU COME TO LAB: Read the section in Munowitz on
Colligative Properties, pp 418-420.
 Define MOLALITY (m)
 Define freezing point
 Define solvent
 Define solute
 Define phase
 Define equilibrium
When a collection of identical molecules or atoms are in the liquid state, they
jiggle about in constant, random motion. The molecules are attracted to one
another; that’s what holds them together in a liquid. They are closer together
than in the vapor, or gas phase, but their thermal motion keeps them farther
apart than if they were in the solid phase1. Some molecules have enough energy
to get into the gas phase.
When a collection of identical molecules freezes, the molecules are still in motion (though not as
much as in the liquid phase…they are colder, and have lost energy). In fact, they
move so slowly, and have so little energy, that the attractive forces between them
predominate over the energy of thermal motion, and the molecules settle down
into an ordered, solid array. Even so, some molecules still have enough energy to
bounce out into the vapor, or gas, phase. The temperature at which a pure liquid
freezes is, of course, its freezing point. At this temperature, solid and liquid are in
equilibrium.
1liquid
water, more dense than ice, is an exception to this rule.
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When a solute is dissolved in the pure liquid, and the temperature is dropped to the freezing
point of the pure liquid, the liquid will not freeze. The solute particles disrupt the liquid molecules from settling
down into an orderly solid array. The system must get colder, & lose more energy, before the solvent freezes.
This phenomenon is called “freezing point depression”, and it is a colligative property. The more particles of
solute you add to the solvent, the more disrupted the orderly array of solvent molecules is, and the colder things
have to get before the solvent freezes.
This property comes in very handy, really. Every winter when the road crews salt the roads, they are they are
adding more solute (NaCl or CaCl2) to a pure solvent (water, in the form of ice and snow). This lowers the
freezing point of water, the ice melts, and with luck and careful driving you don’t fly off the road.
Those of you who aren’t Californians use this property as a method for preventing the liquid in your car radiator
from freezing. Ethylene glycol (C3H6O3) is a typical antifreeze solute. Dissolve it in radiator water, and the
water in your engine won’t freeze at the first frosty day.
Mother Nature uses this property to keep the
blood of Antarctic fishes, and the whites of
penguin eggs, from freezing. “Antifreeze
glycoprotein” is a special protein which
disrupts ice structure and keeps blood
circulating and penguin eggs from turning
into golf balls.
Before you come to lab…
2003 Lake Champlain Penguin Plunge: Feb 15th, -5oF.
Every year folks
http://www.furrs.org/images/plunge2003/default.htm
with an interesting sense of fun leap
through holes in the ice into Lake
Champlain, or in the Atlantic Ocean on New Year’s Day. If the air temperature and wind-chill at both the
Burlington Pier(lake, fresh water) and Boston Harbor(ocean, salt water) are the same, who is enduring greater
cold in the water? Why?
If you put a bottle of Poland Springs pure water and a bottle of seltzer (A solution of
CO2 and water) in the freezer, which will freeze first? Why?
This property also comes in handy during hot summers when your thoughts turn to hand-cranked ice cream. If
you add lots of solute (rock salt, NaCl) to a pure solvent (crushed ice) you lower the freezing point of the ice,
and it gets cold enough to freeze the cream.
This property also comes in handy in doing science. Notice that the drop in freezing point of a pure solvent
depends on how many particles of solute you add. “Particles”, you muse, thoughtfully. “Hmmm. Particles,
moles. Moles, particles. They are related. I’ll bet the next thing that happens is that we convert to moles.” Of
course we do. Life is good.
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The equation for the freezing point depression of a solvent, upon addition of a solute, is:
ΔTf = kfm
ΔTf is the freezing point depression, the difference between the temperature at which a pure solvent freezes
(Tsolvent) and the temperature at which the solution freezes (Tsol’n). ΔTf = Tsolvent – Tsol’n.
m is a concentration unit, molality. Look it up!
Kf is the freezing point depression constant. Different solvents (water, ethanol, cyclohexane, acetone, etc) all
have their own unique Kf, and some of them are listed below. Kf has units of oC . kg solvent/mole solute.
Freezing Point Depression Constants
Compound
Water
acetic acid
benzene
p-xylene
Naphthalene
cyclohexane
carbon tetrachloride
Camphor
Freezing Point (oC)
0
16.66
5.53
13.26
80.29
6.54
-22.95
178.75
kf (oC . kg solvent/mole solute)
1.853
3.90
5.12
4.3
6.94
20.0
29.8
37.7
Oh, look, moles appear! Terrific. And, as you know, getting the mass of something in grams is easy. In this
lab we will do that, and also get enough data pull out moles. If you know grams and moles of something, you
can easily calculate molecular weight, which is what we will do.
In summary: in this lab, we will:
1.
2.
3.
4.
Determine the freezing point of pure cyclohexane;
Add known masses of benzophenone to cyclohexane, and determine the freezing point of the solution;
Graph the data;
Determine the molecular weight of benzophenone.
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THE EXPERIMENT:
You will need:
200 mm Test tube + cork
stopper
Alcohol thermometer
Analytical balance
Plastic beaker to tare
500 mL Beaker
(constant temperature
baths)
Copper stirrer
25 mL graduated cylinder
A watch with a second
hand or seconds display
1
ea
1
2
DEMO SET UP:
500 mL beaker, 200 mL test tube, Cu
stirrer and thermometer.
1
1
1
Cyclohexane
Benzophenone
Rock salt
Ice
Acetone for cleaning glassware
15 g/student pair
6 g/student pair
100 g/student
1 bin/bench
2 squirt bottles/bench
Caution: Cyclohexane and benzophenone aren’t good to eat or drink. Wash your hands after
use. Cyclohexane is flammable. No open fires in the lab!
THE PROCEDURE:
Before you come to lab: Make an Excel Template of the data chart on
page 8.
PUT IT ON A FLOPPY DISK & BRING IT TO LAB. Obviously you won’t include data, but DO
include formulas for calculations. For example, cell d4 is data that you fill in later, but in cell d5 you can type
in “=d4 +b4”. Cell F4 is data, but you can type “=b4+d4+f4” into cell f5.
BRING YOUR KNIGHTCARD TO LAB!!!!
PRELIMINARY GROUP DYNAMIC: MAKE SURE THAT EITHER YOU OR YOUR
PARTNER HAS A WATCH. IF NEITHER OF YOU DO, CHECK OUT A LAPTOP
NOW. Click on “start”  “accessories”  “clock”.
1. SET UP YOUR ICE BATH and WARMING BATH. Fill a 500 mL beaker with ice. Measure the
temperature of the ice bath. Now add a scoop (~ 100-200 g) of rock salt or calcium chloride. Measure the
temperature of the ice-salt bath. RECORD YOUR DATA IN YOUR NOTEBOOK! In your write-up, explain
the difference in temperature between the ice bath and the ice-salt bath. Fill your other 500 mL beaker with
warm tap water. Rinse off and dry the thermometer.
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2. OBTAIN CYCLOHEXANE AND RECORD MASS.
A. Obtain about 20 mL of cyclohexane from the stock bottle in the hood. Take it over to the
balance garage and weigh it to the nearest 0.1 mg (+/- 0.0001 g) in your previously tared large
test tube. Record the mass of the cyclohexane, and cork it up straight away. Cyclohexane is
volatile, and the more cyclohexane evaporates, the greater error you’ll get. You will use this
cyclohexane, and only this cyclohexane, for the rest of the experiment.
3.
DETERMINE THE FREEZING POINT OF PURE CYCLOHEXANE.
A. Now you are back at the bench with your weighed and recorded test tube of cyclohexane. Put
the test tube of cyclohexane into the warm water bath for now. Obtain an alcohol thermometer. Be sure
you remove the protective plastic doohicky protecting the tip of the thermometer, and set the doohickey
in a safe place where you can find it again. Put the thermometer in the test tube of cyclohexane together
with the copper stirrer (see set up on front bench). RECORD THE TEMPERATURE IN YOUR
NOTEBOOK.
B. Freeze the cyclohexane. Put the test-tube containing the cyclohexane, the thermometer and the
stirrer into the ice-salt bath. Record the time. One of you stir the cyclohexane, carefully
observe what it is doing, and note the temperature every time your partner says “30 seconds”.
Look for, notice, and write down at what time you first start seeing solid. Look for, notice, and
write down at what time about half the cyclohexane is solid. Look for, notice and write down at
what time all the cyclohexane is frozen solid. Record the time and the temperatures until the
cyclohexane is frozen solid and for about 2 minutes afterward.
C. Warm up the cyclohexane and freeze it again. Put the test tube of frozen cyclohexane in the
warm water bath and thaw it. Now SWITCH ROLES WITH YOUR LAB PARTNER. The
timekeeper should now OBSERVE AND RECORD. The OBSERVER SHOULD NOW CALL
TIMES. “Why bother?” you might ask. Finding a freezing point can be a little tricky. You are
looking for that temperature at which solid and liquid are in equilibrium. You need to watch for
the first sight of solid crystals. You need to know what the system is doing as it gets solid. You
need to LOOK. Looking, of course, is a subjective exercise. Your eyes tend to see what your
mind expects to see. (if your mind doesn’t know what to expect, sometimes you don’t see
anything at all!). By getting freezing point data for cyclohexane twice, with two observers, your
data will be more reliable. BE SURE YOU WRITE DOWN what you see, and when and what
temperatures.
D. Determine the freezing point of pure cyclohexane with greater precision by graphing time
(x-axis) vs. temperature (y-axis). The
freezing point will be that temperature
where the graph flat lines, as shown on the
sample graph. RECORD YOUR
OBSERVATIONS ON YOUR GRAPH!
First solid cyclohexane
Cyclohexane
observed.
freezes solid
Freezing pt. of pure
cyclohexane
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3. E. If your 2 freezing points are significantly different, repeat the experiment again. Your results
really depend on getting a reproducible freezing point for the pure solvent.
4. MAKE A SOLUTION OF A KNOWN MASS OF BENZOPHENONE WITH YOUR
CYCLOHEXANE AND DETERMINE ITS FREEZING POINT.
A. Thaw your frozen cyclohexane in the warm water bath.
B. While this is going on, obtain some benzophenone and weigh about 0.2 g of it to the nearest
0.1 mg (+/-0.0001 g). RECORD THAT MASS!
C. Put your benzophenone into the cyclohexane and stir it until it is all dissolved.
D. Determine the freezing point of your SOLUTION, as in part 3. Do it at least twice, with two
different observers. Make CAREFUL OBSERVATIONS CORRELATED WITH TIME AND
TEMPERATURE. WRITE THEM DOWN IN YOUR NOTEBOOK.
STIR THE SOLUTION CONSTANTLY AND BRISKLY.
NOTE: This is a somewhat trickier process than determining the freezing point of the pure
cyclohexane. . The pure solvent temperature flat lines as the solvent freezes. You won’t observe
that with the solution. As solvent freezes, the concentration of solute in the unfrozen liquid
increases. The temperature continues to drop. Your data, when graphed, may look something like
this:
First solid observed
Solution all solid
Freezing
point of
solution.
Temp. at
which the 2
lines
intersect.
If your data looks similar to this, you are doing things right.
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What you want to get is the temperature at which the solution just starts to freeze. You can get a
good estimate by drawing a best-fit straight line (by eye) down the points obtained BEFORE you
saw solid cyclohexane, and a best-fit line (also by eye) through. the data points obtained while the
solution was freezing. Where the lines intersect is the freezing point. Be sure to include your
observations on your graph.
If you didn’t stir your solution thoroughly, your data, when graphed, may look like this:
If your data looks like a series of steps, DO THE
EXPERIMENT AGAIN! Stir harder.
Freezing point of the
benzophenone-cyclohexane
solution
5. MAKE A MORE CONCENTRATED SOLUTION AND REPEAT THE EXPERIMENT.
A.
B.
C.
6. CLEAN UP!
A.
B.
C.
Thaw your benzophenone-cyclohexane solution again. While this is going
on, go to the balance and weigh out another 0.1 g of benzophenone to the
nearest 0.1 mg (+/- 0.0001 g). Record the mass.
Add the benzophenone to your benzophenone-cyclohexane solution.
Calculate and record how many grams of benzophenone are now in solution
(First mass + second mass).
Repeat step 4. Determine the freezing point of this solution at least twice, with
two different observers. You may have to add more salt to your icebath to bring
the temperature down enough. If you do, record the temperature of the icebath.
Is it in fact getting colder? (Remember to rinse off and dry your thermometer
before you stick it back in your solution! Make notes of what you observe, and
when. Record your data, graph your data.
Your ice-salt bath goes in the labeled “Slush Bucket”. Rinse and return both
500 mL beakers.
The benzophenone solution gets disposed of in the Organic Waste Jug.
Your test tube and thermometer should be rinsed with acetone. Dispose of the
rinsings in the Organic Waste jug and return the test tube, the thermometer and
the protective cap.
ALL WASTE FROM THIS EXPERIMENT GOES IN THE JUG LABELLED “Cyclohexane and
Benzophenone”, or”Nonhalogenated Organic Waste” IN THE HOOD.
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Dealing with the data:
You will need to work with your recorded mass of cyclohexane, in kilograms, your freezing points,
from your graphs and observations, and the mass of benzophenone you added:








Since you did each trial twice, calculate the average of your freezing points.
Calculate the mass of cyclohexane you used in kg.
Calculate the total amount of benzophenone in each solution, in grams.
Calculate the average freezing point of your pure cyclohexane.
Calculate the freezing point depression for your solutions:
Use the formula ΔTf = Tf pure solvent – Tf sol’n
Use the formula ΔTf = Kfm to calculate the molality of your 2 solutions
You know ΔTf and Kf, ; solve for m.
The units of molality (m) are moles solute (Benzophenone)/kg solvent (cyclohexane.
Using the kg cyclohexane and the molality, calculate MOLES of benzophenone.
You now have moles benzophenone, you have grams of benzophenone, calculate the
molecular weight.
Look up the molecular weight & structure of benzophenone in the Merck Index, REFERENCE IT
CORRECTLY OR LOSE 50 PTS, and calculate the absolute error in MW.
Also draw the structure of benzophenone.
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Your lab will include:
Thorough observations;
An excel spreadsheet of your data ;
6 graphs, with:
appropriate best fit lines;
observations incorporated;
appropriately labeled axes, titles and units;
Freezing points correctly indicated;
An excel spreadsheet of your results, as outlined above.
Include a SUMMARY and ANALYSIS of your data.
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