LABORATORY ACTIVITY:
STUDENT VERSION
Activity 2: The Freezing Point Depression of Lauric Acid
Introduction
Dissolved solute particles disrupt the crystallization process and lower
the freezing point of the solvent. The amount the freezing
point is depressed is directly proportional to the number of moles of particles
dissolved.
Purpose
To determine the freezing point of lauric acid and to observe the effect
a solute has on the freezing point.
Safety
1. Wear protective goggles throughout the laboratory activity.
2. Proper laboratory safety procedures must be followed.
3. The burners heat metal and glass, so proper care must be taken not to
burn yourself. Hot and cool glass look alike.
4. Any substances spilled on you should be immediately washed off with
large amounts of water and the incident reported
to your teacher.
Procedure
Part I: The Freezing Point of Lauric Acid
1. Prepare one hot water
bath (65 °C) and one warm water bath (30 °C). Set up two ring stands
and
place 400 mL water into each 600-mL beaker; using the burner, heat one
bath to 65 °C and warm the
other bath to 30 °C.
2. Weigh the test-tube (25
x 200-mm) empty; fill the test-tube to a height of about 5 cm with solid
lauric
acid and weigh the test-tube and lauric acid.
3. Clamp the test-tube with
the lauric acid in the hot water bath and allow the acid to melt.
4. When the lauric acid
has just melted, remove the test-tube from the water bath and place the
thermometer and stirrer into the lauric acid.
5. Place the test-tube containing
the melted lauric acid, thermometer and stirrer into the 30 °C water
bath and clamp the test-tube in place. Stir the lauric acid and record
the temperature every 30 seconds
until the temperature of the lauric acid has dropped to 40 °C. CAUTION:
Do not attempt to remove or
even move the thermometer or stirrer from the solidified lauric acid!
Part II: The
Freezing Point Depression of Lauric Acid
6. Weigh 2.5 g benzoic
acid.
7. Remelt the lauric
acid as you did in Step 3 above, and as soon as the lauric acid is melted,
dissolve the
benzoic acid in the lauric acid. Take care when adding the benzoic acid
to make sure that all of it
adds to the lauric acid, and that none of it sticks to the sides of the
test-tube. You will need to stir the
mixture vigorously with your stirrer in order to get the benzoic acid to
dissolve completely.
8. Place the
test-tube containing the melted lauric acid, benzoic acid, thermometer
and stirrer into the 30
°C water bath and clamp the test-tube in place. Stir the mixture and
record the temperature every 30
seconds until the temperature of the mixture has dropped to 36 °C.
9. Melt the
mixture in the hot water bath and carefully remove the thermometer and
stirrer from the
mixture. Quickly wipe the thermometer and stirrer with a few layers of
paper towels before the mixture
has had a chance to solidify onto the thermometer.
10. Dispose of the paper
towels and the test-tube contents as directed by your teacher, and return
all
materials to their proper places.
11. Thoroughly wash your
hands before leaving the laboratory.
Data Analysis
1. Graph the temperature vs. time for each trial.
2. What is the freezing point of lauric acid? Explain your reasoning.
3. What is the freezing point of the mixture of lauric acid and benzoic
acid? Explain your reasoning.
4. How many moles of benzoic acid were used?
5. Calculate the molality (mol of benzoic acid/kg of lauric acid) of the
solution.
6. Calculate the change in the freezing point temperature in going from
the pure solvent to the solution.
7. Calculate the freezing point depression constant (K f ) and
compare this with class results. Kf =
change in freezing point divided by molality of solution. Remember that
molality is moles of solute per
1000 g of solvent.
Implications
and Applications
1. Is the freezing point the same as the melting point for pure lauric
acid? Why?
2. Explain what is happening to the energy removed from the lauric acid
in each part of the graph for the
cooling of the pure lauric acid.
3. What is happening, on the molecular level, during the time when the
graph of the pure lauric acid is a
flat, straight line?
4. Why is the graph of the pure lauric acid different from the graph of
the mixture of acids?
5. What is happening, on the molecular level, during the time when the
graph of the lauric acid plus
benzoic acid shows a minimal temperature change with time?
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