Determining the Molar Mass of a Compound Using Freezing Point Depression.
Purpose: Investigate the uses and properties of freezing point depression.
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Setup station using digital thermometer to one decimal point.
Weigh empty test tube and rack.
Add ~4 grams of BHT to test tube, record exact value.
Clamp test tube and insert stirrer and thermometer.
Put test tube into water bath.
Heat water up until temperature change causes BHT to melt.
Remove test tube from water and observe test tube.
Record temperature of freezing point.
Put test tube in water and melt/freeze 4 more times, record each freezing point.
Heat test tube to above melting point and remove thermometer and stirrer, while melted.
Add ~0.5 g Stearic Acid, record exact value.
Find new freezing point using previous method, again using 5 trials.
Data:
Mass of Test tube and Rack ______
Mass of BHT _____
Mass of Stearic Acid ____
BHT Freezing Point Trial 1_____ Trial 2_____ Trial 3_____ Trial 4_____ Trial 5_____ Avg:_____
Mixture Freezing Point Trial 1_____ Trial 2_____ Trial 3_____ Trial 4_____ Trial 5_____ Avg:_____
Calculations:
1. Using the averages determine the change in temperature resulting from the addition of the
solute.
2. If the Kf is 9.2 oC/m, what is the molality of the solute?
3. Using your data determine the moles of solute.
4. Using your data determine the molar mass of Stearic Acid.
5. The actual molar mass of Stearic Acid is 284.49 g, determine your % Error.
Analysis:
1. What impact does the Kf have on the freezing point and what are the advantages of using a
solvent with a large Kf?
2. What impact would having solute stuck on the inside of the test tube above the mixture have on
your results?
3. Thinking about improving your equipment, how could you improve the data you collected.
4. Assuming the same masses, how would water’s freezing point be impacted by the addition of
Stearic Acid, lower or higher, Why?