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Molar Mass Determination from Freezing Point Depression

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Experiment 5 Colligative Properties - Molar Mass
Determination from Freezing Point Depression
Prior Reading
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch15/colligative.html
Background
When a pure solid is dissolved in a solvent, several physical properties of that solvent
change in a way that depend only on the relative amounts of the solute and the solvent
present. Such properties are termed colligative properties. There are four common
colligative properties: boiling point elevation, freezing point depression, vapor pressure
reduction, and osmosis. Colligative effects are observed in many phenomena such as
the melting of ice on winter roads from “salting”, addition of antifreeze to a car’s
radiator to prevent “freeze up”, and the rising of water in trees, due to osmotic pressure.
In ‘salting’, salt is placed on icy roadway and the freezing point of the water will be
lowered. As a result, the ice will melt, depending only on the amount of salt that has
been added.
Pure water freezes at 0 C. If 1 mol of dissolved particles, molecules, or ions is added to
1 kg of water, the freezing point of water drops from 0 C to -1.86 C. This drop is
called the freezing point depression constant for water. Other liquids will have different
constants. The relationship between the concentration of solute particles and the
freezing point depression is expressed as
T = kfp m
where T is the change in the freezing point, kfp is the freezing point depression
constant, and m is the molality of the solution. Molality is defined as the number of
moles of solute in one kilogram of solvent.
Different solutes affect the freezing point depression in different ways. One mol
of a molecular substance will lower the freezing point of 1 kg of water by 1.86 °C. Ionic
compounds when dissolved in water produce more than one mole of particles (ions).
One mol of NaCl will lower the freezing point of 1 kg of water by 3.72 °C, twice as much.
This is because NaCl will dissociate in water to form Na+(aq)and Cl-(aq) ions. Therefore
twice as many particles are present in solution. CaCl2 produces three moles of
particles.
Some representative values are shown below.
Solvent
water
benzene
camphor
stearic acid
kb (°C kg/ mole)
0.52
2.53
40.0
Experiment 5: Colligative Properties
kf (°C kg/ mole)
1.86
5.10
5.95
4.50
1
One use of colligative properties is the determination of the molecular weight of a
substance. In this experiment, the freezing point of pure stearic acid will be measured.
A weighed quantity of an unidentified sample will be added to the pure stearic acid and
the freezing point re-determined. The difference between the freezing points of pure
stearic acid and the mixture is the freezing point depression, ΔTf. Knowing the molal
freezing point constant for stearic acid, the molecular weight of the unidentified sample
can be determined by using the following equations.
m olality =
T f (so lve nt) - T f (m ixtu re)
(E q n . 1 )
k f (solven t)
m oles of u nkn ow n ad de d = (m olality o f un kno w n )(kg solven t use d)
(E q n . 2 )
to ta l gra m s o f un kno w n u sed
m olecu la r w eigh t =
(E q n . 3 )
m oles of u nkn ow n ad de d
Hazards
Stearic, lauric, myristic and palmitic acids are non-hazardous, but prolonged skin
contact may cause irritation. If fatty acids are spilled on skin, students should wash
affected areas thoroughly with soap and water. 2-propanol is flammable and should be
kept away from an ignition source.
The purpose of implementing this procedure is to reduce waste and use less toxic
materials. Previously, compounds such as p-dichlorobenzene, biphenyl, benzil, and
naphthalene were used. As these compounds are toxic and therefore potentially
hazardous, they have been replaced with naturally occurring non-toxic fatty acids. One
of the green objectives of this lab is to reduce waste by converting the used fatty acids
into useful products such as soap, candles or bio-diesel fuel. For this reason, waste
disposal directions should be followed carefully.
Prcedure:
1) Prepare an insulating jacket by wrapping a piece of paper towel around an 18 x 150
mm test tube, A, and fitting it in a 25 x 150 mm test tube, B. Remove the 18 x 150
mm test tube , A, and reserve the 25 x 150 mm test tube, B, and the paper towel as
the insulating jacket (see Figure A). The insulating jacket prevents premature
cooling due to contact with the skin or other surface.
2) Determine the mass of the 18 x 150 mm test tube removed from the insulating
jacket on an analytical balance.
3) Fill the test tube approximately ¾ full, about 9 grams, with stearic acid and reweigh
the test tube and its contents to determine the exact amount of stearic acid
employed.
4) Prepare a hot water bath by filling a 600 ml beaker ¾ full with tap water and
heating with either a Bunsen burner or a hot plate. If a Bunsen burner is used, the
beaker should be supported on a ring stand with wire gauze. The beaker should
also be supported using a chain clamp.
5) Immerse the 18 x 150 mm test tube containing the fatty acid sample in the hot
water bath to melt the fatty acid. After the fatty acid sample has completely melted,
place the thermometer in the fatty acid sample and heat until the sample reaches
85 °C. From this point on, the thermometer is not removed from the fatty acid
Experiment 5: Colligative Properties
2
sample to prevent loss of material and contamination of bench tops with fatty acids.
Remove the test tube from the water and dry the outside.
6) Place the 18 x 150 mm test tube containing the fatty acid sample in the previously
prepared insulating jacket. Stirring constantly with the thermometer, record the
temperature of the sample every 30 seconds for 8-10 minutes. Temperatures are
collected until the temperature of the sample remains constant, changing by less
than 0.1 °C per reading, for 3 minutes, 6 readings. Perform a second trial using the
same sample.
7) To the fatty acid sample used above, add approximately 1 g of an unidentified
sample. Exactly 1 g is not needed, but you must know the mass added to the
nearest 0.1 mg.
8) Repeat steps 5 and 6 on this fatty acid sample. Two trials of this sample are
performed.
9) To the same fatty acid sample, an additional 1 g of the same unidentified sample is
added. Again, exactly 1 g is not needed but you must know the mass added to the
nearest 0.1 mg.
10) Repeat steps 5 and 6 on this fatty acid sample. Two trials of this sample are
performed.
T h e rm o m e te r (0 -1 1 0 C )
Paper Tow el
1 8 x 1 5 0 m m (in n e r tu b e -A )
2 5 x 1 5 0 m m (o u te r tu b e -B )
F a tty A c id M ixtu re
Figure A. Schematic for the construction of an insulating jacket.
Clean-up procedure
1) After you have completed your final trial, use the hot water bath to reheat the test
tube and the fatty acid sample to 85 °C until all of the fatty acid has melted.
2) Pour all of the melted fatty acid mixture out into a clean waste container labeled
“PRIMARY FATTY ACID WASTE”.
3) At this point, you must shut off your Bunsen burner due to the flammability of the 2propanol to be used in the next step.
4) Fill the test tube 3/4 full with 2-propanol, place it in the hot water bath and stir the
mixture with the thermometer to dissolve all of the residual fatty acids deposited on
the sides of the thermometer and test tube walls.
5) Once the sample is dissolved, pour the 2-propanol mixture into the waste bucket
labeled “SECONDARY FATTY ACID WASTE”.
6) Repeat steps 4) and 5) once or twice more until the test tube is completely clean.
Data Treatment
Experiment 5: Colligative Properties
3
To accurately determine the freezing points of pure stearic acid and each of the
solutions, “cooling curves” of temperature (y-axis) versus time (x-axis) are plotted. Data
where the temperature changes by more than 0.5 °C per 30 seconds is plotted as one
series and data where the temperature changes by less than 0.5 °C per 30 seconds as a
second series. A best fit line is then calculated for each series and the freezing point is
obtained by finding the intersection of the two best fit lines, see Figure B.
T e m p (°C )
90
85
80
F re e z in g P o in t
75
70
65
0
200
400
600
T im e (s )
Figure B. Example of colligative properties data obtained using a mixture of stearic and
myristic acids. The figure illustrates the data analysis process for determining the
freezing point of the mixture.
Three graphs are needed. The two trials for each sample may be recorded on the same
graph. From these graphs determine the freezing point for the pure stearic acid and
each of the stearic acid and unidentified sample mixtures. The plot for pure stearic acid
will be the same as the plots for the mixtures except the melting point will appear at a
higher temperature for the pure stearic acid then it does for the mixtures.
OPTIONAL
Converting the Fatty Acid Waste to Soap
Conversion of the fatty acid waste stream to soap is accomplished as follows: 10
g (39 mmol assuming 100% stearic acid) of fatty acid mixture is placed in a 600 mL
beaker. The fatty acid is heated until completely melted. Applied heat should not
exceed 85 °C to prevent boiling of the subsequently added water/base mixture. The
melted mixture is magnetically stirred, and 19.5 mL (39 mmol) of 2 M sodium hydroxide
is added over 20 minutes in 5 mL aliquots. Stirring is adjusted as needed to avoid
excess sudsing. The solution is stirred at 75 °C for an additional 5-10 minutes to aid
removal of excess water. The warm mixture is transferred to a food grade mold such as
an ice cube tray, candy mold, or a large container lined with plastic wrap. The mixture
is then cured for at least 2 weeks in a well-ventilated area. After 2 weeks, the soap
should be checked for hardness. Generally, the soap will reduce in volume to
approximately 1/3 of the original volume. Once the soap is hard enough to remove
from the mold, it is ready for use as a general-purpose laboratory soap for washing
hands or glassware. The soap we have prepared has not been found to be irritating, but
monitoring the pH during the neutralization reaction is recommended to prevent
addition of excessive amounts of base. A pH of 8 - 9 for the final mixture is
recommended. Further experiments to add emollients such as glycerin, to prepare the
soap in flake form specifically for washing glassware, and to change the base from
NaOH to KOH to investigate the preparation of a liquid soap are planned. The soap is
not recommended for washing faces.
Name
Experiment 5: Colligative Properties
4
Calculations - Lab 5:
Colligative Properties
1) Average the freezing points for each of the three samples tested. Find the change in
freezing points, ΔTf, for the mixtures by comparing the freezing point of each to the
freezing point of pure stearic acid.
2) Use this to find the molality of each solution.
3) Use the molality to determine the moles of unidentified sample added in each trial.
NOTE: For the second addition of unidentified sample you are finding the total
number of moles added.
4) Determine the molecular weight of the unidentified sample.
5) Determine the average molecular weight and identity of your unidentified sample
using the list below.
Myristic Acid
C14H28O2
Palmitic Acid
C16H32O2
Lauric Acid
C12H24O2
Experiment 5: Colligative Properties
5
Name
Pre-Lab 5:
Colligative Properties
1. The normal freezing point of benzene is 5.5 C. A benzene solution contains 62.5 mg
of naphthalene (MW = 128 g/mol) per 1.00 g benzene. The solution freezes at 3.0 C.
What is the molal freezing point constant for benzene?
2. In the wintertime, it is common to sprinkle NaCl on icy roads and driveways. Why is
this done? Would calcium chloride be more or less effective? Please explain.
3. A 0.2346-g sample of an unknown substance was dissolved in 20.0 mL of
cyclohexane. The density of cyclohexane is 0.779 g/mL. The freezing point depression
was 2.5 C. Calculate the molar mass of the unknown substance. (kf = 20.0 C/m)
4. What safety rules must be observed during this experiment?
Experiment 5: Colligative Properties
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