OBJECTIVES/OVERVIEW

When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of the 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 solute itself. Equation (1) below shows this relationship:

ÐT = K_fm

where ÐT is the freezing point depression, K_fis the freezing point depression constant for a particular solvent (2.72°C kg/mol for lauric acid in this experiment), and m is the molality of the solution.

Molity has units of	mol solute
	kg solvent

In this experiment, you will first find the freezing temperature of the pure solvent, lauric acid (dodecanoic acid), CH₃(CH₂)₁₀COOH. You will then find the freezing point of a mixture of lauric acid and an unknown solute, such as acetic acid, CH₃COOH. By measuring the freezing point depression, ÐT, and knowing the masses of solute and solvent, you can use equation (1) above to find the molecular weight (molar mass), in g/mol, of the unknown solute.

Your instructor will assigning students to work together in teams of two. In order to work efficiently, students should divide the work in such a way that efforts are not being duplicated. For example, one partner can set up the beaker of water while the other partner weighs the lauric acid. Both partners should observe the temperature changes as the substance cools.