CHEM 362 L EXPERIMENT 10
Cryoscopic Determination of Molecular Weight
Introduction
This experiment shows one method of determining the molecular
weight of a compound. The student will
measure the depression of the freezing point of a solvent by addition of known
weights of solute. The observed depression of the freezing point, combined with
the accurately measured weight of the solute (and that of the solvent, obtained
from its volume)can be used to determine the molecular
weight of the solute. The solvent used will be cyclohexane.
The students will first measure a series of cooling curves for a known solute
(naphthalene), and then repeat the procedure for an unknown solute.
Procedure
Follow the procedure described in the laboratory text (1),
with the following exceptions: you will determine the molecular weight of both
naphthalene and the unknown compound, and you will continue adding 0.6 g
pellets to the solution until the observed freezing point is off the
thermometer scale. You will be using a Beckman thermometer, which is designed
to read temperature differences, rather than actual temperatures. The cyclohexane should be close to its freezing point before
the thermometer is placed in the solution, otherwise the mercury may leave the
working column.
Calculations
Using Equation 21, calculate the molecular weight of the solute for each of the solutions. A plot of M versus DTf can be used to extrapolate to infinite dilution. Report the molecular weight of the solutes. The unknown solute is a hydrocarbon; determine its molecular formula based on the observed molecular weight.
(1).Shoemaker, D. P.;