Freezing Point Depression

Each student worked alone and performed, in the course of an afternoon, two tedious cooling curve measurements (cyclohexane and a cyclohexane solution) by means of a mercury thermometer and a clock. They would later construct graphs of the data, determine the freezing point depression of the solution, and calculate the solute's molecular weight.

With MeasureNet, a cooling curve can be measured and printed in just a few minutes, so we have completely revamped the experiment. We prepare a number of solutions before lab using one or two solvents, including cyclohexane, two or three solutes, and several concentrations. These known solutions are divided among the student pairs on each network (three or four solutions per pair). The students measure cooling curves and determine the freezing point of each.

In a relatively short time, the group determines a large number of freezing points which are combined into a single graph of freezing point versus molality. This graph is printed and discussed in lab to enable the students to discover the factors affecting solution freezing point. Finally, each student prepares an unknown solution and performs a cooling curve measurement to get a molecular weight from the freezing point depression (to retain some level of individual accountability in the experiment). The group approach results in a much different laboratory experience for students than was the case earlier with each student working in isolation.