Determination of Molecular Weight by Freezing-Point Depression
AP Chemistry
When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of
moles of solute particles added. This property, known as freezing-point depression, is a colligative
property; that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance
itself. The equation that shows this relationship is:
∆T = i • Kf • m
where ∆T is the freezing point depression, i is the van’t Hoff factor (1 in this lab), Kf is the freezing point
depression constant for a particular solvent, and m is the molality of the
solution.
In this experiment, you will first find the freezing temperature of a pure
solvent, a nonpolar compound with the common name butylated
hydroxytoluene. This compound is abbreviated BHT and is frequently
used as an antioxidant in foods. The IUPAC name for the compound is
2,6-di-tert-butyl-4-methylphenol. Its structural formula is shown at the
right.
Temperature
(ºC)
You will then add a known mass of naphthalene solute, C10H8, to a known mass of BHT, and determine
the lowering of the freezing temperature of the solution. By measuring the freezing point depression, ∆T,
and the mass of naphthalene, you can use the formula above to find the freezing point depression constant
of BHT. By repeating this step with an unknown solute, and using the molality, the molar mass of the
unknown can be determined.
Time of Cooling
Materials & Equipment:
Butylated hydroxytoluene, BHT, C15H24O (s)
Naphthalene, C10H8 (s)
Unknown substance
(2) utility clamps, large ring & ring stand
Laptop, LabPro, Logger Pro software, USB cable
Temperature probe
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(2) 18 x 150-mm test tubes
(2) 400-mL beakers
Wire stirrer & test tube holder
Sensitive balance
Hotplate
(2) Scoopulas
Procedure:
1. Obtain and wear goggles. Aprons are recommended.
2. Prepare the computer for data collection by opening the Experiment 15 file from Chemistry with
Computers. Then open the experiment file that matches the probe you are using. The vertical axis
of the graph has temperature scaled from 0°C to 100°C. The horizontal axis has time scaled from 0
to 10 minutes.
Part I: Freezing Temperature of Pure BHT
3. Add about 250 mL of tap water to a 400-mL beaker. Place the beaker on the hotplate and turn it to
the highest setting.
4. Accurately measure about 5 grams of BHT into the test tube using a sensitive balance. Record the
exact mass.
5. Clamp the test tube in the water bath and insert the temperature probe and stirring wire into the
BHT. Use the small clamp to hold the probe. About 30 seconds are required for the probe to
warm up to the temperature of its surroundings and give correct temperature readings. Heat the
BHT to about 90oC in the water bath.
6. Click Collect to begin data collection. Raise the test tube out of the hot water bath and observe the
BHT temperature as the melted BHT cools. It is important to continuously stir the BHT to
maintain even cooling. Stirring will also help prevent supercooling.
7. Make a note of the temperature where crystals begin to form. To determine the freezing
temperature of pure BHT, you need to determine the mean temperature in the portion of graph
with nearly constant temperature. Move the mouse pointer to the beginning of the graph’s flat
part. Press the mouse button and hold it down as you drag across the flat part of the curve,
selecting only the points in the plateau; Note: it will not display a box around the part you are
selecting. Click on the Statistics button, . The mean temperature value for the selected data is
listed in the statistics box on the graph. Record this value as the freezing temperature of BHT.
Click on the upper-right corner of the statistics box to remove it from the graph.
Part II: Freezing Temperature of a Solution of Naphthalene and BHT
8. Measure about 0.6 gram of naphthalene onto a piece of weighing paper and record its mass, then
add the naphthalene to the test tube containing the same BHT from Part I. Heat the mixture in the
hot water bath until the substances are all melted. Stir well to be sure that the solution is
homogenous.
9. To prepare the computer for data collection, choose Store Latest Run from the Experiment menu.
This stores the data so it can be used later. To hide the curve of your first data run, click the
Temperature vertical-axis label of the graph, and uncheck the Run 1 box. Click OK .
10. When you have completed Step 9, click on the Examine button, . To determine the freezing
point of the BHT-naphthalene solution, you need to determine the temperature at which the
mixture initially started to freeze. Unlike pure BHT, cooling a mixture of naphthalene and BHT
results in a gradual linear decrease in temperature during the time period when freezing takes
place. As you move the mouse cursor across the graph, the temperature (y) and time (x) data
points are displayed in the examine box on the graph. Locate the initial freezing temperature of
the solution, as shown in the graph on page 1. Record the freezing point.
11. Disposal: Place the test tube back in the hot water bath until the mixture re-melts. Pour the melted
substances out onto crumpled newspapers or paper towels.
Part II: Freezing Temperature of a Solution of Unknown and BHT
12. Repeat the Part II procedure using a new sample of BHT and about 0.6 gram of the unknown
compound. Dispose of the mixture appropriately.
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Data: Record all the necessary data and organize it into a data table.
Graphs:
• Parts I and II: Temperature vs. Time of BHT and BHT-naphthalene mixture
• Parts I and III: Temperature vs. Time of BHT (same data) and BHT-unknown mixture
Label your graphs like that on page 1.
To print a graph of temperature vs. time showing two data runs:
1. Click the Temperature vertical-axis label of the graph. To display both temperature runs, check
the Run 1 and Latest boxes. Click OK .
2. Click File, Print, then enter your names in the footer. Please print only one copy of the graph.
Analysis:
1. Determine the difference in freezing temperatures, ∆T, between the pure BHT and the mixture of
BHT and naphthalene, and calculate the molality of the BHT-naphthalene mixture
2. Calculate the experimental Kf, the freezing point depression constant, for BHT.
3. Calculate the molecular mass of the unknown, and the percent error once the unknown is known.
Extension:
Here is another method that can be used to determine the freezing temperature from your data in Parts II
and/or III. With a graph of the Part II (or III) data displayed, use this procedure:
1. Move the mouse pointer to the initial part of the cooling curve, where the temperature has an
initial rapid decrease (before freezing occurred). Press the mouse button and hold it down as you
drag across the linear region of this steep temperature decrease.
2. Click on the Linear Regression button, .
3. Now press the mouse button and drag over the next linear region of the curve (the gently sloping
section of the curve where freezing took place). Press the mouse button and hold it down as you
drag only this linear region of the curve.
4. Click
again. The graph should now have two regression lines displayed.
5. Choose Interpolate from the Analyze menu. Move the mouse pointer left to the point where the
two regression lines intersect. When the small circles on each cursor line overlap each other at the
intersection, the temperatures shown in either examine box should be equal to the freezing
temperature for the naphthalene-BHT mixture.
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