Molar Mass by Freezing-Point Depression
Purpose: Use freezing-point depression to determine the
molar mass of benzoic acid.
Materials:
Weigh cup
Large test tube
Test tube cap
Test tube rack
Lauric acid
Benzoic acid
Computer
USB link
Temperature sensor
400 mL beaker
Hot plate
Ring stand
Buret clamp
Scoopula
Beam balance
Background Information:
• Kf lauric acid = 3.90 oC/m
• Accepted freezing point for lauric acid = 44.0oC
• Molar mass of benzoic acid = 122 g/mol
Procedure:
Part One
1.
2.
3.
Construct a data table that will allow you to record the
mass of lauric acid (solvent); mass of benzoic acid
(solute); freezing point of lauric acid; and freezing
point of benzoic/lauric acid solution.
One person in the group, turn on the computer.
Connect the temperature sensor to the USB link, and
then connect the USB link to the computer. Launch EZ
Screen.
Another person in the group, fill a 400-mL beaker with
approximately 300 mL of water and begin heating on
the hot plate. Use the temperature sensor to measure
the temperature of the water. Click on the start
4.
5.
6.
7.
8.
9.
button.
You want your water to reach
approximately 90oC. Do not let the water boil at any
time.
While waiting for the water to warm, mass 8 grams of
lauric acid, record the mass, and transfer the acid to
the test tube.
Clamp the test tube to the ring stand with a buret
clamp. Lower the clamp/tube assembly until the test
tube is in the water, but not touching the bottom of the
beaker.
Heat the lauric acid in the water bath until it melts.
Move the clamp/tube assembly out of the water.
Place the temperature sensor in the liquid lauric acid.
Stir gently with the temperature sensor until the
temperature reaches 60oC.
When the temperature reaches 60oC, exit out of EZ
Screen and launch Data Studio by double clicking on
Then click on
the icon in the bottom right tray.
.
10. Click on the start button.
Continue collecting
data until the temperature reading remains constant for
about 2 minutes. You should see crystals of lauric acid
forming in the solution; you have reached the freezing
point of the lauric acid. Click on the stop button.
11. Print enough copies of the graph for everyone in the
group. Exit out of the program.
12. Move the buret clamp/test tube/temperature sensor
back into the hot water bath and liquefy the lauric acid
once again.
13. Carefully remove the temperature sensor from the
liquid lauric acid. Allow any liquid drops of acid to fall
back into the test tube. Remove the temperature
sensor and wipe it clean with a paper towel.
Part Two
1.
2.
3.
4.
5.
6.
7.
Obtain 1 gram of benzoic acid. Place the benzoic
acid in the test tube that contains the lauric acid
from Part One.
Heat the benzoic acid/lauric acid mixture until it
liquefies.
Launch EZ Screen again.
Repeat steps 6-13 from Part One for the benzoic
acid/lauric acid mixture.
Pour the liquefied solution into the waste container in
the back of the room. Warning: Do not pour the
liquid solution down the sink! It will solidify
and clog the drain.
Clean up your area and wash your hands.
Go back to the classroom area and fill in your data
table using your graphs.
Calculations:
1.
Calculate the change in freezing point (ΔTf) when the
benzoic acid was added to the lauric acid.
2.
Calculate the molality (m) of the solution.
3.
Using the formula for molality, calculate the moles of
benzoic acid used.
4.
Using the mass of benzoic acid used and the answer
to #3, calculate the experimental molar mass of
benzoic acid.
5.
Using the accepted value for the molar mass of
benzoic acid, calculate your percent error.