Korea Advanced Institute Science and Technology

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Exp 6. Using Freezing-Point Depression to Find
Molecular Weight

• Korea Advanced Institute Science and Technology
• General Chemistry Laboratory II

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Procedure

• Obtain and wear goggles.
• Plug both an interface and a computer to main
  power.
• Connect a temperature probe to channel 1 of the
  Vernier computer interface.
• Connect the interface to the computer with the
  proper cable.

Press this power button
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5. Start the Logger Pro program by double
clicking on the desktop.
6. Open the file 04 Freezing Point from the
Advanced Chemistry with Vernier folder. We will
utilize a template for this experiment.
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As soon as you arrives at the lab, prepare a hot
water in a 500-mL beaker on a hot plate in
advance. Prepare two clean, well-dried test
tubes by washing inside of it with acetone and
exposing to the heat of a hair dryer. Weigh
about 8g of lauric acid for PART I and add it
into a test tube by using a glass funnel. Mix
about 8g of lauric acid with about 1 g of
benzoic acid for PART II , and then add them into
another test tube by using the funnel. (You must
record the precise masses of the lauric acid and
benzoic acid.) Melt the contents of the two test
tubes with a holder or a utility clamp
simultaneously in the hot water of the beaker.
(You need to shake the test tubes to let them
melt completely. Be careful not to spill or stain
the hot contents inside wall of the test tubes. )

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Part 1. Determine the Freezing Temperature of
Pure Lauric Acid

• Add about 300ml of tap water with a temperature
  of 20-25C to a 500ml beaker.

2. Insert the temperature probe into the melted
lauric acid. Fasten the utility clamp to the ring
stand or fasten the test tube to the utility
clamp so the test tube is above the water bath.
Click Collect to begin the data collection.
(CAUTION Be careful not to spill the hot lauric
acid on yourself and do not touch the bottom of
the test tube)
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3. Lower the test tube into the water bath. (Make
sure the water level outside the test tube is
higher than the lauric acid level inside the test
tube.)
4. With a very slight up-and-down motion of the
Temperature Probe, continuously stir the lauric
acid for the ten-minute duration of the
experiment. (CAUTION Be careful not to touch
the bottom of the test tube)
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5. When the data collection is complete, use a
hot water bath to melt the lauric acid enough to
safely remove the Temperature Probe. Carefully
wipe any excess lauric acid liquid from the probe
with a paper towel or tissue.
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6. Freezing temperature Determination 1) Move
the mouse pointer to the beginning of the graphs
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 plateau. 2)
Click the Statistics button. 3) The mean
temperature value for the selected data is listed
in the statistics box on the graph. 4) Record
this value as the freezing temperature of pure
lauric acid. 5) Click on the upper-left corner
of the statistics box to remove it from the
graph.
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Part 2. Determine the Freezing Temperature of a
Solution of Benzoic Acid and Lauric Acid

• Prepare the computer for data collection.
• 1) From the Experiment menu, choose Store Latest
  Run.
• 2) To hide the curve of your first data trial,
  click the Temperature y-axis label of the graph
  and choose More.from the dropdown list.
• 3) Uncheck the Run1 Temperature box and click OK.

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2. Perform the experiment with the test tube
containing a pre-melted solution with 1g of
benzoic acid dissolved in 8g of lauric acid. 3.
Repeat steps 2-5 in Part. 1
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4. The Freezing Temperature Determination The
freezing temperature of the benzoic acid-lauric
acid solution can be determined by finding the
temperature at which the mixture initially
started to freeze. 1) Click and drag the mouse
to highlight the initial part of the cooling
curve where the temperature decreases rapidly
(before freezing occurred). 2) Click on the
Linear Regression button. 3) Now click and drag
the mouse over the next linear region of the
curve (the gently sloping section of the curve
where freezing took place). 4) Click the Linear
Regression button again. The graph should now
have two regression lines displayed. 5) Choose
Interpolate from the Analyze menu. Move the
cursor to the point of intersection of the two
lines you will know if the cursor is over the
intersection when the temperature readings
displayed in the Interpolate box are the same.
This is the freezing point of the benzoic
acid-lauric acid mixture. 6) Record the freezing
point in your data table.
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• Store your results safely in a folder of
  GenChemII on the desktop.

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You must attach your results (data and
graph) to your laboratory report. Start an
Excel program to store on your portable USB
memory or send a mail as a file. Click and drag
the mouse to highlight the data part. After
copying the data by pressing the key CtrlC,
paste it on the Excel file with CtrlV.
Follow the same method for the graph copy. Or
you can copy a graph showing both trials.
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Waste Disposal Instructions Dispose of all
contents (lauric acid itself and the mixture of
the lauric acid and benzoic acid) of the melted
state in the designated plastic bottle in the
fume hood. It is easy to dispose of the melted
liquid !!!! If the test tube has solids,
wash them with acetone to remove completely.