Question

PLZ! I need all questions in the procedure of Molecular Weight Determination by Freezing Point Depression lab

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

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 or temperature probe in the distilled water. Take time-temperature data at time interval of your choice until freezing point is determined.   It may not be necessary to freeze the entire sample. Make sure to write qualitative observation along the way. If a temperature probe is used, follow the procedure given at the end of the procedure to store the data into Jackie’s USB.

   * 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 0.15 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 ~0.15g of unknown from the ice bath. Allow the mixture to melt.

9. Add another portion of ~0.15g of same unknown (now ~0.30g 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.

Note: If the temperature probe is used, this is how you save your data onto Jackie’s USB. The folder will be linked to the class website.

Answer 1

I'll answer some of your questions, because we are not allowed to answer multiple questions. Some of your questions, I actually need the data you obtained to do so. However I'll try to answer some questions of both parts, and the rest of the questions, post them in another question thread:

Part 1:

1. a) It's not neccesary, because you can measure the volume of the distilled water in a graduate cylinder and then pass it to the test tube. Assuming water density to be 1 g/mL you are measuring the mass of water. However if you don't measure it in a graduate cylinder, then, indeed will be neccesary to weight the test tube before and after the load.

b) Mostly the time because a large amount of water, will take longer to freeze than a small amount.

2. a) The freezing point predicted for water (at standard conditions) will have to be 0 °C.

b) Salt lowers the freezing point of water. Salt added to crushed ice creates a slush that will stay liquid at temperatures well below the freezing point of water. In other words, it will help to the ice to last longer.

Part 2.

6. a) it will be lower from the freezing point of water, because adding another sample to the water, will cause that those molecules of unknown replace another molecules of water, therefore, it will need to release more energy to freeze. therefore the freezing point will be lower and below of 0 °C.

Hope this helps