1. Freezing Point Depression Experiment
GOAL: Determine the molecular weight of an unknown substance using freezing point depression.
1) Determine the FP temperature of a pure solvent.
2) Determine the FP temp of a solution made by dissolving a nonionic, nonvolatile unknown substance.
3) Calculate the molecular weight of the unk solute.

2. Low Down on Experiment
Solvent – para-xylene: freezes between ºC
Solute – unknown: will lower freezing point of para-xylene when in solution.

3. Freezing Point Depression Experiment
Set up the apparatus as shown on page 5.
1) Weigh cork and test tube.
2) Add ~30 mL p-xylene to test tube.
3) Reweigh cork / test tube / p-xylene.
4) Calculate mass of p-xylene used.

4. Data Table I for Pure Solvent
Mass of test tube and solid cork
45.879g
Mass of test tube and solid cork and p-xylene g
Mass of p-xylene g
74.123g – g = g p-xylene

5. Time/Temp Data
Freeze the pure p-xylene solvent. A device known as Vernier will gather time vs. temperature data throughout the experiment. The data will be transferred to YOUR USB memory jump drive to be used in generating a freezing point depression graph for the pure p-xylene solvent.

6. Results Table III (fake data)
Freezing point of p-xylene, Tfº
13.3 ºC
Freezing point of solution, Tf
Value for ∆Tf
Molality of unknown solution, m
Mass of p-xylene (in grams) g
Mass of p-xylene (in kilograms) kg
Moles unknown solute
Mass of unknown added to test tube
Molecular Weight of unknown solute

7. Data Table II for Liquid Unknown
Unknown letter D
Mass of test tube and solid cork g
Mass of test tube and solid cork and liquid unknown
66.511g
Mass of liquid unknown
2.410 g
66.511g – g = g liquid unknown

8. Time/Temp Data
Freeze the solution (unknown solute added to p-xylene solvent) The same Vernier device will again gather time vs. temperature data throughout the experiment. The data will again be transferred to YOUR USB memory jump drive to be used in generating a freezing point depression graph for the resulting solution.

9. Results Table III (fake data)
Freezing point of p-xylene, Tfº
13.3 ºC
Freezing point of solution, Tf
11.1 ºC
Value for ∆Tf
Molality of unknown solution, m
Mass of p-xylene (in grams) g
Mass of p-xylene (in kilograms) kg
Moles unknown solute
Mass of unknown added to test tube
2.410 g
Molecular Weight of unknown solute

10. Freezing Point Depression Experiment
Calculations:
ΔTf = (FP of pure p-xylene - FP of p-xylene solution)
ΔTf = 13.3 oC – 11.1 oC = 2.2 oC
ΔTf = (Kf) m
2.2 oC = (4.30oC/m) m
0.51 m = molality of the unknown p-xylene solution

11. Results Table III (fake data)
Freezing point of p-xylene, Tfº
13.3 ºC
Freezing point of solution, Tf
11.1 ºC
Value for ∆Tf
2.2 ºC
Molality of unknown solution, m
0.51 m
Mass of p-xylene (in grams) g
Mass of p-xylene (in kilograms) kg
Moles unknown solute
0.014 mole
Mass of unknown added to test tube
2.410 g
Molecular Weight of unknown solute

12. Freezing Point Depression Experiment
Calculations:

13. Results Table III (fake data)
Freezing point of p-xylene, Tfº
13.3 ºC
Freezing point of solution, Tf
11.1 ºC
Value for ∆Tf
2.2 ºC
Molality of unknown solution, m
0.51 m
Mass of p-xylene (in grams) g
Mass of p-xylene (in kilograms) kg
Moles unknown solute
0.014 mole
Mass of unknown added to test tube
2.410 g
Molecular Weight of unknown solute
1.7 x 102 g/mole