Molecular Weight Determination by Freezing Point Depression

Objective:

Determine the molecular weight of unknown compound using freezing point depression.

Use the “LAB : HOW TO…” link from the class website if you need help with how to use balance, Bunsen burner… and such.

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Procedure:

Note : You may see super cooling. The freezing point is the maximum temperature after supercooling or after first crystal appears.

Part I: Freezing Point of Pure Water

1.  Place distilled water in the test tube. Determine the mass of the water used.

* Do you need to weigh the test tube?

* The amount of water should not matter since we are measure ring the freezing point which is intensive. Name one major disadvantage of using a large sample of water for this lab.

2. Prepare an ice bath in a foam cup with ice and table salt. Place the cup in a beaker to give it more stability. The ice bath should be deep enough so that it is above the level of the water in the test tube but well below the top. Take care not to let any of the salt or ice get into the sample of distilled water.

* What is the predicted freezing point of water?

* Why do you think you need to put salt in the ice bath?

3. Place a thermometer in the distilled water. Record the initial and final temperature. It may not be necessary to freeze the entire sample. Make sure to write qualitative observation along the way.

* Draw a section of the theoretical cooling curve you are expecting for this part of the lab.

* If the entire sample is not frozen, how would you know that you have reached the freezing point?

4. Do not discard the sample of the distilled water, because the sample will be used in Part II.

Part II: Molecular Weight of the Unknown

5. Remove the test tube containing the distilled water from the ice bath. Allow the ice to melt.

6. Weigh out approximately 0.10 gram of unknown sample. Add the sample to the distilled water, and stir until it is all dissolved. Return the test tube to the ice bath. Insert the thermometer or temperature probe.

* What is the predicted freezing point of this solution compare to water?

* Name one advantage by putting a large amount of unknown at this point.

* Name one advantage by putting a small amount of unknown at this point.

7. Take time-temperature data as in Part I. Again, the sample does not have to be frozen solid in order to determine the freezing point. Make sure to write qualitative observation along the way.

8. Remove the test tube containing the distilled water and ~0.10g of unknown from the ice bath. Allow the mixture to melt.

9. Add another portion of ~0.10g of same unknown (now ~0.20g of unknown) and repeat the rest of step 6 and 7.

* What is the predicted freezing point of this solution compare to the solution from (6)?

*Name one procedural manipulation you might want to do at this point if the solution from (6) took a very long time to freeze. Explain.

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Prelab Questions :

(PL1) Define colligative properties.

(PL2) Write the mathematical equation that you would use this lab.

(PL3) “i” is called the van’t Hoff factor. What does it represent?

(PL4) Should your equation from (PL2) include “i”? Briefly explain.

(PL5) What is the molar mass of X (i=1) if 50.00g of water containing 2.50g of X has a freezing point of -11.5C?

(PL6) Using information from PL5, recalculate the molar mass of X if i=2.

(PL7) Compare your answers from (PL5) and (PL6). Briefly explain.

(PL8) How is melting point different than freezing point?

(PL9) List the following in the increasing melting points. Pure water, 1.0M sugar water, 1.0M salt water.

(PL10) Draw a data table you will be filling in during the lab. Include all the measurements that will be made in this lab and qualitative observation if needed.

Analysis :

All unknowns are assumed to have i=1 unless given otherwise during the lab.

(A1) Calculate the molar mass of your unknown using delta T between experimental fp of pure water and fp with ~0.10g of unknown.

(A2) Calculate the molar mass of your unknown using delta T between experimental fp of pure water and fp with ~0.20g of unknown.

(A3) Calculate the average molar mass of your unknown.

Questions :

(Q1) What would happen to the final result (what is final result?) if a significant amount of water evaporated after measuring its mass in step(1)?

(Q2) What would happen to the final result if your lab partner added ~2.0g (Instead of ~0.15g) of the unknown? You did not know.

(Q3) What would happen to the final result if your unknown actually has i=3?

(Q4) Why is it not necessary to wait for the entire sample of water to freeze in order to determine its freezing point?

(Q5) Why is it a good idea to measure the freezing point of the water instead of assuming that its freezing point is exactly 0o C?

Summary :

Kf for water ______

Experimental freezing point of water ______

Theoretical freezing point of water ______

Unknown number ____

solution (6) : freezing point of mixture is ______containing ______g of unknown

solution (9) : freezing point of mixture is ______containing ______g of unknown

Molar mass of unknown ______

Abstract:

No more than 100 words stating why the lab was done, what was done, the major result(s), and conclusion(s).

Reflection Statement:

One thing I might do differently next time is …… because…….

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