Freezing-Point Depression to Determine an Unknown Compound

Overview

Source: Laboratory of Lynne O' Connell — Boston College

When a solid compound is dissolved in a solvent, the freezing point of the resulting solution is lower than that of the pure solvent. This phenomenon is known as freezing-point depression, and the change in temperature is directly related to the molecular weight of the solute. This experiment is designed to find the identity of an unknown compound by using the phenomenon of freezing-point depression to determine its molecular weight. The compound will be dissolved in cyclohexane, and the freezing point of this solution, as well as that of pure cyclohexane, will be measured. The difference between these two temperatures allows for the calculation of the molecular weight of the unknown substance.

Procedure

A temperature probe interfaced to a computer is used to acquire the temperature readings in this experiment. The temperature probe has an uncertainty of ±0.1 °C.

1. Setting the Parameters in the Software

Set the length of the experiment to 800 s.
Set the sampling rate to 1 sample per second.
Set the upper limit for the temperature range to 40 °C and the lower limit to 0 °C.

2. Measuring the Freezing Point of Cyclohexane

Results

The mass of cyclohexane that was dispensed can be calculated. The density of cyclohexane is 0.779 g/mL.

Equation

The values for T_f° and T_f can be determined from the plots.

The molar mass, and thus the molecular weight, of the

Application and Summary

Perhaps the most visible application of the phenomenon of freezing-point depression occurs during the winter months, when roads and sidewalks become icy, and salt is used to treat the slippery surfaces. When the salt mixes with the ice, the freezing point of the water is depressed so the ice melts at a lower temperature. Because the degree of the freezing point depression is dependent on the number of particles in solution, salts that release three ions per formula unit, such as calcium chloride (CaCl₂), are o