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
Also see MELTING ICE
RETURN to SYLLABUS
You will do the following:
- 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.
- 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.
- 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
- From the above three masses and three
temperatures you have enough data to determine the Molecular Weight of the UNKNOWN at two different concentrations.
|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
- 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.