1 | P a g e Using Freezing-Point Depression to Find Molecular Weight When a solute is dissolved in a solvent, the freezing temperature 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 nature of the substance itself. The equation that shows this relationship is: T= Kf • mwhere Tis the freezing point depression, Kfis the freezing point depression constant for a particular solvent (3.9°C-kg/mol for lauric acid in this experiment1), and mis the molality of the solution (in mol solute/kg solvent). 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. Calculate the freezing point depression of the mixture Calculate the experimental molecular weight of benzoic acid. Calculate the percent error Figure 1 MATERIALS LabQuest2 with App utility clamp Temperature Probe 18 150 mm test tube 400 mL beaker lauric acid ring stand benzoic acid 1 “The Computer - Based Laboratory,” Journal of Chemical Education: Software , 1988, Vol. 1A, No. 2, p. 73.