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Experiment 14 Determination of Molar Mass by Freezing Point Depression

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**1. **Experiment 14 Determination of Molar Mass by Freezing Point Depression

**3. **Background – Colligative Properties Adding a nonvolatile solute to a solvent changes the properties of the solution. Compared to the pure solvent, the solution will have:
a lower freezing point
a higher boiling point
The amount of the change is a function on the number of solute particles that are in the solution.
These changes can be calculated using the Van’t Hoff equation: DT = i(Kf)m

**4. **Background – Van’t Hoff Equation DT = i(Kf)m
DT – freezing point depression
i – Van’t Hoff factor
Kf – freezing point depression constant
KfC – finger lickin’ good
m – molality of the solution

**5. **Background – Van’t Hoff Equation DT = i(Kf)m
DT – freezing point depression
This is the difference between the freezing point of the pure solvent and the freezing point of the solution.

**6. **Background – Van’t Hoff Equation DT = i(Kf)m
i – Van’t Hoff factor
The Van’t Hoff factor describes the number of bodies a molecule of solute forms on dissolution. For nonelectrolytes, the factor has a value of 1.

**7. **Background – Van’t Hoff Equation DT = i(Kf)m
Kf – freezing point depression constant
This value in unique to the solvent. For our experiment, Kf is 20 oC-kg-mol-1 for cyclohexane.

**8. **Background – Van’t Hoff Equation DT = i(Kf)m
m – molality of the solution
Molality is a concentration definition

**9. **Background – Van’t Hoff Equation DT = i(Kf)m
m – molality of the solution
Molality is a concentration definition

**10. **Experimental Procedure There are two main procedures to this experiment
Set-up the computer.
Collect freezing point data (cool!).

**11. **Experimental Procedure – Setup

**12. **Experimental Procedure – Collecting Data Freezing Point of Cyclohexane
Weigh out approximately 15mL of cyclohexane into a 25 x 150 mm test tube. Obtain a two-hole rubber stopper and insert the temperature sensor into one of the holes. Place the test tube into a test tube clamp hooked to a ring stand.
Put an ice bath (use a 400 mL or 600 mL beaker) underneath the supported test tube.
3. Immerse the test tube into the ice bath and keep stirring the cyclohexane until the timer reaches 600 seconds (10 minutes).

**13. **Experimental Procedure – Collecting Data

**14. **Experimental Procedure – Collecting Data Freezing Point of the Unknown Solution
Allow the frozen cyclohexane to thaw (it is acceptable to use your hands to let the solvent thaw quicker). Weigh out 0.100g of unknown solute.
Add the solute to the liquid cyclohexane and mix well to dissolve all of the solute in the cyclohexane.
Begin the second run by repeating steps 2 - 3 of the previous procedure (freezing point of cyclohexane) to record the data.
Allow the frozen cyclohexane to thaw. Weigh out 0.100g of unknown solute.
Repeat steps 2 - 3 of this procedure.
Once you have finished all three runs, plot temperature vs. time for each of the runs and use the graphs to determine the freezing point of each solution.

**15. **Experimental Procedure – Data, Solvent

**16. **Experimental Procedure – Data, Solvent

**17. **Experimental Procedure – Data, Solution

**18. **Experimental Procedure – Data, Solution

**19. **Experimental Procedure – Data Analysis

**20. **Experimental Procedure – Data Analysis

**21. **Experimental Procedure – Data Analysis

**22. **Experimental Procedure – Data Analysis

**23. **Experimental Procedure – Data Analysis

**24. **Experimental Procedure – Data Analysis

**25. **Experimental Procedure – Data Analysis

**26. **Experimental Procedure – Data Analysis

**27. **Experimental Procedure – Data Analysis

**28. **Experimental Procedure – Data Analysis

**29. **Experimental Procedure – Data Analysis

**30. **Safety Avoid breathing fumes of cyclohexane.
Cyclohexane is flammable; no open flames are permitted in the laboratory.

**31. **Waste Disposal All solutions from this experiment are to be placed in the container labeled “Recovered Organics Without Halogens”.