Melanie Mickens
February 21, 2014
Statement of Purpose: The purpose of this lab was to determine the freezing temperature of the pure solvent, lauric acid, as well as finding the freezing temperature of a mixture of lauric acid and benzoic acid. Along with that, the freezing point depression of a mixture was to be calculated and along with the molecular weight of benzoic acid.
Background:
Colligative properties are properties which depend only on the number of solute molecules present in a solution and include freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure. The equation is ∆T=Kf *m

Procedure:
For part 1, add around 300 mL of tap water with a 20-25 C temperature into a 400 mL beaker, then putting it on the base of the ring stand. Use the utility clamp to get a test tube with the hot, melted lauric acid in it. Put the temperature probe in the lauric acid and secure the utility clamp to the ring stand. Lower the tube into the water making sure the water level outside the test tube is higher than the lauric acid level on the inside. Stir the acid up and down for ten minutes. After the data collection, use a hot water bath to melt the lauric acid enough to remove the temperature probe. Find the freezing temperature by mean temperature on the graph that had a fairly constant temperature. For Part 2, get a test tube that has the melted solution in it. After that, repeat the steps stated above starting at the part that says fastening the test tube to the clamp and lowering it into the beaker. After ten minutes, use the graph to find the freezing point of the solution.

Data:

Mass of lauric acid
7.46 g
Mass of benzoic acid
1.32g
Freezing Temperature of pure lauric acid
43.13°C
Freezing Point of the benzoic acid-lauric acid mixture
36.63°C

Analysis of Data:

• Molality
T = Kf *m
T = 43.13°C-36.63°C
T=6.5°C
6.5=(3.9 °C kg/mol)(m) m= 1.67 mol/kg

• Moles of Benzene m=moles of Benzene/kg lauric acid
1.67=moles of Benzene /.00746
Moles of Benzene=.0125 moles

• Experimental Molar Mass mm= mass benzoic acid/moles benzoic acid mm= 1.32g/.0125 moles mm= 106g/mol

• Actual Molar Mass
C7H6O2
C=12.011*7
H=1.0079*6
O=16.00*2
122.1244g/mole

• Percent Error
Actual – Expected /Actual *100
106-122.1244* 100 122.1244
Percent Error=13.2 %

Application of Results to a Larger Context: One potential source of error is in determining the freezing point. Although for lauric acid the average of the portion of the graph when the solution was freezing was used, determining which part of the graph to use was difficult. Another source of error concerned the solutions. Although great care was taken to prevent any solution from leaving the test tube, when the temperature probe was melted out of the solid acid, slight traces of acid remained on the probe and had to be removed with a paper towel.