Experiment 13
Molecular Weight Determination
by Freezing Point Depression
Purpose
• The purpose of this experiment is to use
the freezing-point depression of a
solution of an unknown compound in
para-dichlorobenzene (PDB) to
determine the molecular weight of the
unknown.
Introduction
• What are colligative properties?
• Examples of colligative properties
Vapor pressure over a solution
Boiling-point elevation
Freezing-point depression
Osmotic pressure
•
The first three colligative properties
are closely related.
• Situation: solute B dissolved in solvent
A.
• If the solution is ideal (not all are!), the
vapor pressure over the solution follows
Raoult’s Law.
Raoult’s Law
• PT = PAoXA + PBoXB
•
•
•
•
•
PT: vapor pressure of the solution
PAo: vapor pressure of pure A
PBo: vapor pressure of pure B
XA: mole fraction of A in the solution
XB: mole fraction of B in the solution
PT = PAoXA + PBoXB
• If solute B is nonvolatile (PBo =0), PBoXB
is zero.
• Raoult’s Law becomes PT = PAoXA.
• Since XA is less than 1.00, PT must be
less than PAo.
Phase Diagram Pure A
Pressure (atm)
2
1
0
270
280
290
300
310
320
Temperature (K)
330
340
350
Phase Diagram---Solution of B in A
Pressure (atm)
2
1
0
270
280
290
300
310
320
Temperature (K)
330
340
350
Phase Diagram---Solution of B in A
Pressure (atm)
1.25
1
0.75
320
322
324
326
328
330
332
Temperature (K)
334
336
338
340
Phase Diagram---Solution of B in A
Pressure (atm)
1.25
1
0.75
280
281
282
283
284
285
286
Temperature (K)
287
288
289
290
• Note boiling-point elevation and
freezing-point depression.
• These temperature changes depend on
the composition of the solution.
• ΔT = k m
FPD equation ΔT = kf m
• ΔT = Tf(solution) – Tf(solvent)
• kf: freezing-point depression constant
for the solvent (-7.10 o/m for PDB)
• m: the molality of solute (moles solute/
kg solvent)
Safety
• Aprons and safety glasses
• Bunsen burners: keep hair, clothes,
paper away.
• PDB and unknowns are not soluble in
water; they are also flammable and/or
toxic. USE WASTE BOTTLES, NOT
THE SINK, FOR DISPOSAL.
Safety 2
• Thermometers are fragile and
expensive; handle with care!
• Do not attempt to remove the
thermometer from a solidified sample.
Remelt sample, then remove
thermometer.
Safety 3
• After experiment is over: remelt
sample; remove thermometer and
stirrer; pour molten sample into waste
container. Then use a small amount of
acetone to rinse any remaining material
in test tube into waste container.
Procedure
• Work in pairs.
• Needed equipment: Ring stand, clamp,
ring, wire gauze, Bunsen burner, 1000mL beaker, largest test tube (25 x 200
mm).
• Check out thermometer-stirrer-stopper
assembly from stockroom.
• Weigh test tube to nearest 0.01 g; record
mass on data sheet. Add 20-24 g of PDB to
test tube; weigh again and record mass on
data sheet.
• Weigh two portions of unknown, each about
1.5 grams (to nearest 0.01 g). Record
masses and unknown number on data sheet.
• Set up water bath and burner (p. 13-6); clamp
test tube in bath. When PDB melts, fit
thermometer-stirrer assembly. Remove test
tube from bath and allow to cool. Stir
continuously.
• Begin measuring when temperature reaches
60oC. Record temperature every 30 seconds.
Note when solid first appears. After 8
minutes, stop collecting data.
• Do not attempt to remove thermometer now!
PDB cooling curve
60
temp (oC)
59
58
57
56
55
54
53
52
0
2
4
time (min)
6
8
• Remelt PDB; test tube clamped in bath.
• Remove thermometer assembly.
• Add first portion of unknown.
• Swirl/stir/mix until unknown dissolves.
• Replace thermometer.
• Remove sample from water bath.
• Begin recording temperature as before
when solution has cooled to about
60oC.
• Note temperature at which solid first
appears.
• Stop collecting data after eight minutes.
• Remelt sample. Remove thermometer
assembly.
• Add second portion of unknown (total
mass of solute in this run is the sum of
the two individual masses).
• Dissolve unknown.
• Cool and collect data as before.
• After last run, remelt mixture, remove
thermometer assembly, and pour
mixture into waste container.
• Use small volume of acetone to rinse
remaining material from test tube into
waste bottle.
• Rinse thermometer assembly with
acetone (into the waste bottle) and
return thermometer assembly to
stockroom.
Calculations
• Graph temperature vs time; plot all
three runs on one graph.
Temperature (C)
Cooling curves Expt. 13
60
59
58
57
56
55
54
53
52
51
50
49
48
0
2
4
Time (min)
6
8
• Extrapolate lines to get freezing point
for each run.
• Get DT values for each sample
containing unknown:
DT = Tsolution – Tsolvent
• Calculate molality from DT = kfm
(kf = -7.10 oC/m)
Moles solute = molality x kg PDB
mass_solut e
MW 
moles_solu te
• Average MW values from the two runs.