Molar Mass by Freezing Point Depression Lab

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Nick Boyea
Billy Lee
Molar Mass by Freezing Point Depression
The purpose of this lab is to measure the freezing point depression of a solution of an unknown substance and BHT to determine the molar mass of the unknown substance. Summary of Lab Procedure
If not already completed, crush a small amount of BHT and pack it into a capillary tube. Use a small rubber band to clamp the capillary tube to the thermometer, and fasten the thermometer to a ring stand. Fill a Thiele tube with water, and immerse the bottom of the capillary tube in it. Place the Thiele tube over a Bunsen burner and heat the substance until the BHT melts. As soon as evidence of melting occurs, record the temperature. Repeat the experiment using the BHT and the unknown substance.

Pre-Lab Questions
1. a. 53.02 – 50.78 = 2.24 °C
b. molar mass = __7.1 °C/m X 2.04 g (solute)__
0.0248 kg (solvent) X 2.24 °C
molar mass = 260. g
2. Colligative properties are physical properties of solutions that depend upon the number but not the kind of solute particles present. Data
Melting Points
Pure BHT| 68.8 °C|
BHT + cetyl alcohol| 56.5 °C|
BHT + unknown| 60.2 °C|

| BHT| Cetyl Alcohol| Unknown|
Solution #1 – BHT + Cetyl Alcohol, g| 5.00 g| 1.00 g| –| Solution #2 – BHT + Unknown, g| 5.00 g| –| 1.00 g|

Post-Lab Calculations
Kfp, BHT, °C/m| 4.9 °C/m|
molar mass, unknown, g/mole| 240 g/mole|

Post-Lab Questions
1. a. If the thermometer accidentally read 1.4 °C too high, there would be no effect, because the change in temperature would still be the same as if the thermometer read the correct temperature. b. If some of the solvent was spilled before the solute was added, there would be a lower value because losing solvent would make a higher mole calculation for the solute. This would cause the molar mass to be too low. c. If some of the solute was spilled after it was weighed and before it was added to the...
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