3.Test tube sizes 18X 150 mm, 20X 150 mm, and 25X 150 mm work well.4.In order to have plenty of time to run two trials in one 40-50 minute period, we recommend that you have pre-measured lauric acid and lauric acid-benzoic acid samples in test tubes. In Part I, a sample of lauric acid that is approximately 8 grams (filled to a depth of about 10 cm) works well. In Part II, measure 8 g of lauric acid and 1 g of benzoic acid into each test tube, to the precision of your electronic balance (at least 0.01 g). Place a label on each test tube showing the precise masses of the lauric acid and the benzoic acid. Be sure the label will be above the water level of the water bath. Clear packing tape over the label helps too. Students will record and use these masses in their calculations.5.A hot-water bath on a hot plate adjusted to a low setting (one that maintains the water bath at a temperature of 80-90°C) can be used to melt the lauric acid samples and maintain them in the liquid state. When initially preparing the lauric acid-benzoic acid mixture, the benzoic acidwill not melt in the water bath. It is necessary to “flash melt” the benzoic acid over a Bunsen burner flame. Once the benzoic acid is melt and mixed to form a homogenous solution, the solution is easily melted in the water bath. Place the beaker with samples of lauric acid on onehot plate at one location, and the beaker with samples of the lauric acid-benzoic acid mixtures on a second hot plate at another location. 6.If you follow the procedure in Steps 4 and 5 above, you can easily reuse samples of lauric acidand the lauric acid-benzoic acid mixtures. There should be very little contamination of the samples if you remind students to wipe any excess liquid from the probe at the end of a trial. This method also makes clean-up a great deal easier for you at the end of the lab. Stopper the test tubes and store them for future use. Lab # 761, Freezing Point Depression to Find Molecular Mass, (5/24/07) by Advancing Science Program, Gettysburg College, Gettysburg, PA17325. A Science in Motion Program-6-
Science in Motion7.Step 6 mentions at least one temperature reading of 50°C or greater. This ensures a portion ofthe graph showing the drop down to the 44°C freezing temperature of lauric acid. 8.Have a hot-water bath available to free probes that have become frozen in lauric acid (or the mixture). Students will have to disconnect the temperature probe from the Lab Pro. The test tubes can be immersed in the bath to melt the solution and free the probes. Be sure the wires do not get against the hot plates.9.If you have class periods longer than 50 minutes, you may choose to have students weigh out their own samples of lauric acid and benzoic acid. We still recommend having hot plates with hot water baths available (as well as Bunsen burners and stirring rods) to save students the time to heat up a water bath.
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