Computer 15 Chemistry with Vernier 15 - 1 Using Freezing Point Depression to Find Molecular Weight When a solute is dissolved in a solvent, the freezing point is lowered in proportion to the number of moles of solute added. This property, known as freezing point depression, is a colligative property ; that is, it depends on the ratio of solute and solvent particles, not on the identity of the solute. The equation that shows this relationship is: T = i K f • molal where T is the freezing point depression, K f is the freezing point depression constant for a particular solvent (3.9°C-kg/mol for lauric acid in this experiment 1 ), molal is the molality of the solution (in mol solute/kg solvent), and we assume i is 1 for this molecular solute. In this experiment, you will first find the freezing temperature of the pure solvent (lauric acid, CH 3 (CH 2 ) 10 COOH). You will then add a known mass of benzoic acid solute, C 6 H 5 COOH, to a known mass of lauric acid, and determine the lowering of the freezing temperature of the solution. By measuring the freezing point depression ( T ) and the mass of benzoic acid, you can use the colligative property law shown above to find the molecular weight of the benzoic acid solute in g/mol. OBJECTIVES In this experiment you will: Determine the freezing temperature of pure lauric acid. Determine the freezing temperature of a solution of benzoic acid and lauric acid. Examine the freezing curves for each. Calculate the experimental molecular weight of benzoic acid. Compare it to the accepted molecular weight for benzoic acid. Figure 1 1 “The Computer - Based Laboratory,” Journal of Chemical Education: Software , 1988, Vol. 1A, No. 2, p. 73.
Computer 15 15 - 2 Chemistry with Vernier MATERIALS computer utility clamp Vernier computer interface 18 150 mm test tube Logger Pro lauric acid 2 Temperature Probes benzoic acid 400 mL beaker thermometer ring stand PROCEDURE 1. Obtain and wear goggles. Connect the Temperature Probe to the computer interface. Prepare the computer for data collection by opening the file “15 Freezing Pt Depression” from the Chemistry with Vernier folder.
You've reached the end of your free preview.
Want to read all 4 pages?
- Fall '15
- Chemistry, Freezing-point depression, Lauric acid