Chemistry 150

Please have the following pages ready before class on Thursday, February 21. As usual, please write an abstract and paper-clip it to the front of your individual writeup. The abstract and the carbon-copy pages of the write-up is due in class on Thursday, February 28.

This lab takes advantage of the fact that colligative properties of solutions depend only on the number of moles of solute in a solution and not the nature of the solute. This enables us to determine the number of moles of an unknown material and thus determine its molar mass.

The temperature difference between the freezing point of a pure solvent and a solution made with that solvent is proportional to the concentration of solute. The units of concentration used in this experiment are moles of solute per kg of solvent also called molality. By not having a volume term in the concentration expression we avoid any problems that might arise with volume changes caused by changes in temperature.

In brief terms, the experiment works like this: You melt a pure material then allow it to cool and solidify while recording its temperature. This gives you its freezing point. Next you add a known mass of an unknown material (and, in this lab, you will have a choice of one of three unknowns) to the pure material and make a solution. Again you allow it to cool and record its temperature. The difference in freezing points of the pure material and the solution is directly proportional to the number of moles of solute in the solution. You don't know what the solute is but you do know its mass, and from your experiment, the number of moles. Thus you know its molar mass, in principle.

To be sure, you add some more of the unknown material (a measured mass, of course) to the remelted solution and, once again, you allow it to cool and record its freezing point.

The change in freezing points and the concentration in molality (symbolized by m)are related by the following equation:

DTf = Kf m

Kf is called the freezing point depression constant, and it is constant for a given solvent. Once this is known for a given solvent that solvent can be used as a measuring tool to determine molar mass. We will use the value of K_f = 8.40°C/m for t-butanol.

In order to simplify the experiment you will be given the freezing point depression constant for your solvent. You could determine it on your own but it is difficult to get accurate results from this experiment and it is bad enough using just one set of lousy data. The solvent used in this experiment is tertiary butyl alcohol also called tert-butanol or t-butanol. It has a freezing point close to room temperature and is relatively safe and odor free in comparison to naphthalene, which is traditionally used in this type of experiment.

Section 11.5 of the text, especially sample exercise 11.10, will be quite helpful in preparing for this experiment.

Oh, and one other complication: you are going to write up the procedure as you do the experiment. Exercise 11 and your own observation skills will be invaluable as you write down the steps as you perform them.

Lab 4: Determination of molar mass by freezing point depression

Part 1. Purpose

This should be fairly evident from the preceding introduction. Summarize it into a sentence.

Part 2. Materials and methods

Chemicals list: t-butanol and 3 unknown organic substances

Sketch and label the various pieces of equipment. This picture may be helpful. Note that the beaker will be full of ice. I did not add it so that you could see what was inside.

Here’s a close-up view of the test tube:

You don’t have to stir vigorously, but you do want to make sure that the solution or solvent is of uniform temperature throughout the tube (crystals will of course begin to form at the test tube walls otherwise).

Part 3. Procedure

Do I have to do this every single time? This time you get to do it: using exercise 11 to guide you, write down every single thing you do during this lab in a series of numbered steps. Notice that you should use the past tense as you write these steps down.

Warning: t-butanol and some of the unknowns are quite pungent. If you get any material on yourself, immediately wash it off with soap and water.

Waste disposal: All materials in this lab are to be treated as organic waste; nothing should go down the sink. Please use the beakers in the hoods provided to get rid of waste.

Part 4. Original data and preliminary analysis

You should be able to figure out (from exercise 11, if nothing else) what data you need to collect. Of course, you should write down unknown numbers, unknown identities, and freezing point depression constants in this section.

Part 5. Calculated results

Show the molar mass determination calculation, from the freezing point depression all the way to the molar mass.

Plot the time (x-axis) versus temperature (y-axis) data in order to obtain the freezing points. These graphs are called cooling curves. Do separate graphs for each freezing point. Remember, the solution freezing points are going to be a bit trickier (see exercise 11).

Part 6. Questions

1. a. Explain why it is the freezing point depression of a solvent does not depend on the identity of the solute.

b. Explain why it is the freezing point depression of a solvent does depend on the concentration of the solute.

You may combine a and b into a single answer.

2. Occasionally, a phenomenon called supercooling occurs in the cooling curve. Given its name, explain how you recognize supercooling when it occurs on the cooling curve. Did this happen in your case? Would supercooling make determining the freezing point easier or more difficult? What can you do about it, then?

3. Calculate the percent error between your molar mass and the actual molar mass for both solution cooling curves. Did you have a systematic error? If so, name its most likely source.

4. Calculate how much your molar mass would have changed if your freezing temperature had changed 0.5°C on the less concentrated run. Do the calculation for both 0.5°C higher than you obtained and 0.5°C lower than you obtained. Was it worthwhile to use the digital thermometers, which have an uncertainty of 0.1°C, rather than the alcohol thermometers, which have an uncertainty of 0.5°C?

Part 8. Conclusion

First sentence: “The average molar mass of (“unknown identity”) was __________ g/mol, determined by the freezing point depression of t-butanol.”

Explain how much percent error you had off of the actual molar mass (question 3) and whether you had a systematic error or not. Suggest methods to minimize the systematic error.

How confident are you of your results? In other words, was this a good experimental setup? Question 4 may help here. If your results were not very good, suggest (beyond fixing the systematic errors) other changes that might result in better numbers.

Abstract

In a hundred words or less, summarize the main result of the experiment and the method by which the result was obtained. Briefly discuss the accuracy of the result and what factors may have affected that accuracy.