MOLECULAR WEIGHT
by
Freezing Point Depression

This is one of 7 experiments you do this year which, at least in part, involve determination of a Molecular Weight, Formula Weight or Equivalent Weight

MW by Freezing Point Depression TUTORIAL
Also see MELTING ICE

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This Experiment

You will do the following:

  1. Weigh out some p-dichlorobenzene (p-DCB) and determine its freezing point. p-DCB is the solvent. You are determining the freezing point of the pure solvent.
  2. Add a weighed amount of UNKNOWN to the above p-DCB and again determine its freezing point. The UNKNOWN is the solute and this is the freezing point of the first solution.
  3. Add an additional weighed amount of UNKNOWN to the material in step 2 and again determine the freezing point. This increases the amount of solute present. This is the second solution.
  4. From the above three masses and three temperatures you have enough data to determine the Molecular Weight of the UNKNOWN at two different concentrations.

Helpful Hint

In pouring the unknown from the sample tube into the tube containing the p-DCB, pour carefully. Do not spill.
  • Before pouring make sure the unknown is loose and free flowing.
  • Then tilt the sample tube of unknown slightly downward and roll the tube back and forth with your fingers. The powdered Unknown should roll down the tube in a controlled manner.
  • You should try to add about half for the first solution freezing point but don't worry too much is you add more or less.
  • Remember you need to know the exact amount of UNKNOWN (solute) you added to the p-DCB.
  • Also remember, in the freezing point lowering of the second solution, the p-DCB contains not only the second addition of UNKNOWN but also still contains the first.

R.W. Kluiber 1/28/2000