Lab 3: Determination of molar mass by freezing point depression

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1 Chemistry 162 The following write-up may be inaccurate for the particular chemicals or equipment we are using. Be prepared to modify your materials/procedure sections when performing the exercise. Please have sections 1, 2 and as much of 3 as makes sense ready before class on Wednesday, January 30; also, please bring a flash drive to the lab. As usual, please write an abstract and attach it to the front of your individual writeup. The abstract and the carbon-copy pages of the write-up is due in class on Wednesday, February 6. Lab 3: Determination of molar mass by freezing point depression Introduction: 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: ΔT f = i K f m The constant i is called the van t Hoff factor and must be looked up (cite the reference) K f 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 have to look up the value of Kf from a reputable source. Section 13.7 of the text (Colligative Properties) will be quite helpful in preparing for this experiment.

2 Date Your name, partner s name Lab 3: 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: distilled water, sodium chloride and an unknown organic substance Safety issues: The unknown is quite pungent, so avoid direct inhalation. If you get any of this 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 to get rid of waste. Equipment list: Sketch and label the various pieces of equipment. This picture may be helpful, though you will be using the LabPro datalogger and temperature probe connected to the laptop, rather than a stand-alone probe. You will not be using any t- butyl alcohol. Note that the beaker will be full of salty ice water.

3 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 The following steps are a bare-bones outline of the procedure. Fill in blanks and add more detail to each of the underlined areas in the outline. It is preferred that you should use the past tense as you write these steps down. Explicitly state the criterion you used to determine that you had enough data. In steps 1 to 5, you will determine the freezing point of pure distilled water. You cannot assume that the instruments will state that it is exactly 0 C. The time/temperature curve will reveal some good information. 1. Place distilled water in the (what size?) test tube. Determine the mass of the water used. How?

4 2. Prepare a saltwater ice bath in a foam cup with about g ice and about ml table salt. Place the cup in a (how big?) 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 contaminate the sample of distilled water. 3. Place a thermometer or temperature probe (which one?) in the distilled water. Is there a rubber stopper involved here? Take time-temperature data at rate of samples per second or seconds per sample? until the freezing point is determined. How is this determined? How long did it take overall? It may not be necessary to freeze the entire sample. 4. Do not discard the sample of the distilled water, because the sample will be used in the next steps. Remove the test tube containing the distilled water from the ice bath. Allow the ice to melt. 5. Weigh out approximately 0.2 gram of the unknown sample; record the exact amount. 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 (which one?). 6. Take time-temperature data as with the distilled water. What sampling rate of data acquisition was used? How long did this sample take to freeze? Again, the sample does not have to be frozen solid in order to determine the freezing point. 7. Remove the test tube containing the distilled water and ~0.2g of unknown from the ice bath. Allow the mixture to melt. 8. Add another portion of ~0.2g of same unknown (now ~0.4g of unknown), record the exact amopunt, and repeat the rest of step 5 then step 6. What sampling rate of data acquisition was used? How long did this sample take to freeze? You should store the time/temperature data for each run (write down the names of the files for future reference) as a.txt file that you can to yourself or else store on a flash drive. You will not have access to the dataloggers after lab, so any data you want from them should be stored on a flash drive! Part 4. Original data The table should be set up for at least four runs (distilled water, 0.2 g sample, 0.4 g sample, and an extra column if you need to repeat). The rows should at least include data such as the mass of water, mass of unknown (if applicable), and freezing point, all with appropriate units. Table format discipline is necessary to keep the different trials organized. The van t Hoff factor and the freezing point depression constant value should be recorded in this section, and need to have citations to the reference you used.

5 Part 5. Calculated results Show the molar mass determination calculation for your unknown solute, from the freezing point depression all the way to the molar mass for both trials. Set up the algebraic equation to indicate what you are doing. Plot the time (x-axis) versus temperature (y-axis) data in order to obtain the freezing points; you may do it by hand, by Excel or by printing out the graph from the datalogger. These graphs are called cooling curves, and should have axes labels as well as titles identifying each one. Make separate graphs (or overlay each graph on the same set of axes) for each freezing point. You will show the solution freezing points by adding the necessary lines. Note: You may see the phenomenon of supercooling. The freezing point is the maximum temperature after supercooling or after first crystal appears. Part 6. Group results Enter your group s solute mass and freezing point depression for both trials on the spreadsheet. Plot solute mass (x-axis) versus temperature change (y-axis). Clearly title the graph and label the axes (complete with units). Calculate a best-fit line equation (you may draw the line if you wish) and a correlation coefficient. Note: you must have at least ten points on the graph for these calculations to be meaningful. Calculate the mean and standard deviation of the molar masses for all groups. This means that you will have to do the freezing Comment on any outliers (which may even be your own values!) and state whether they were used in the calculation. Part 7. Questions 1. Explain why it is the freezing point depression of a solvent does not depend on the identity of the solute. 2. Explain why it is the freezing point depression of a solvent does depend on the concentration of the solute. 3. What would happen to the final result if your unknown actually has i=3?

6 4. In the plot of solute mass versus temperature change, what quantity is proportional the slope of the best-fit line? In principle, then, what should the correlation coefficient have been? 5. Calculate how much your molar mass would have changed if your freezing temperature was off by 0.5 C. 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, in terms of precision of the final number? 6. Why is it not necessary to wait for the entire sample of water to freeze in order to determine its freezing point? 7. Why is it a good idea to measure the freezing point of the water instead of assuming that its freezing point is exactly 0 o C? Part 8. Conclusion First sentence: For my team, the average molar mass of ( unknown identity ) was g/mol, determined by the freezing point depression of water. Calculate the percent error between your calculated molar mass and the actual molar mass. Did you have a systematic error? If so, name its most likely source and methods to minimize the systematic error. How confident are you of your results? In other words, was this a good experimental setup? Question 5 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 the standard abstract format and in a hundred words or less, state the class s result for your unknown (use a format like the first sentence of the conclusion), and the experimental method by which that value was obtained, assess the accuracy (% error off the true value) and the precision (the standard deviation) of the class result, and what sources of error the class may have made that affected each.