Experiment 14 Determination of Molar Mass by Freezing Point Depression

Experiment 14 Determination of Molar Mass by Freezing Point Depression PowerPoint PPT Presentation


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Goal. To determine the molecular mass of an unknown covalent solid by determining the freezing point depression of a solution.. Background

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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”.

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