Using Freezing-Point Depression to Find Molecular Weight Formal Lab Report Purpose: The purpose of this lab is to be able to use the freezing point of a substance to find the molecular weight of it. To do this, we will use the Logger Pro program to obtain the freezing point of lauric acid and a solution of benzoic acid and lauric acid. Then, we’ll go calculate the freezing point depression of the solution and the molecular weight of benzoic acid. Procedure Summary: Obtain lab goggles and a lab apron before you begin.
Connect a temperature probe to channel 1 of the vernier computer interface. Connect the other cable to the computer. Engage Logger Pro and open the file titled “04 Freezing Point” in the “Advanced Chemistry with Vernier” folder. Part 1: Determine the Freezing Point of Pure Lauric Acid Fill a 400 mL beaker with 300 mL of 20 - 24 oC tap water and insert it in the ring stand. Use the utility clamp while obtaining a hot test tube containing hot, melted lauric acid from your instructor. Fasten the clamp at the top of the tube.
Put the temperature probe into the hot lauric acid and begin to collect data. Attach the utility clamp to the ring stand so that it’s above water. Lower the test tube in the water to where the water line is above the lauric acid that is inside the test tube. In an up-and-down motion, stir the lauric acid continously with the temperature probe for ten minutes. When you finish collecting data, use the hot water to melt the lauric acid enough to remove the temperature probe from the test tube. Wipe off the excess lauric acid on the probe with a paper towel. Find the freezing temperature.
Part 2: Determine the Freezing Point of a Benzoic Acid and Lauric Acid Solution Get a test tube filled with 1 g of benzoic acid dissolved 8 g of lauric acid. Record the precise masses. Repeat steps 1 to 7. Find the freezing temperature. Record findings in data table. Print graph. Data and Calculations: When putting the lauric acid in into the water, our group noticed that the lauric acid froze almost instantly while the benzoic-lauric acid solution, it took a while to freeze. |Mass of lauric acid (g) |8. 10 g | |Mass of benzoic acid (g) |0. 8 g | |Freezing temperature of pure lauric acid (oC)|42. 97 oC | |Freezing point of the benzoic acid (oC) |40. 43 oC | Conclusions and Questions: At the end of our experiment, we found the freezing point of the benzoic-lauric acid to be , and the molecular weight of the benzoic acid to be . 42. 97 - 3. 9 = 39. 07 = ? T m = 10. 02 mol/kg (m) 8. 10 g lauic acid / 1000 g = 0. 081 mol benzoic acid (12. 011 X 7) + (1. 008 X 6) + (15. 9 X 2) = 122. 123 g/mol C6H5COOH