Ensure the cable is not touching any hot surface. 11. To determine the freezing temperature of pure lauric acid, you need to determine the mean (or average) temperature in the portion of graph with nearly constant temperature. In the “Graph” window, click on the beginning of the graph’s flat part. Press on the screen and hold it down as you drag across the flat part of the curve, selecting only the points in the plateau. Click on the “Analyze” menu → Statistics, check the box next to temperature. The mean temperature value for the selected data is listed in the statistics box on the graph. Record this value as the freezing temperature (T 1 ) of lauric acid. Uncheck the box next to temperature to remove the statistics box from the graph. Part II Freezing Temperature of a Solution of Benzoic Acid and Lauric Acid 12. Weigh about .25g of benzoic acid using a weighing paper. Make sure you tare the balance with the weighing paper. Record the precise mass. When lauric acid completely melted, add the benzoic acid. Stir gently with the temperature probe. Make sure both acids are liquids, if not continue heating. After the solution is melted take out the test tube from the hot bath, still in the clamp, rotate it away from the hot bath and start measuring the temperature by clicking Collect in the program as you did for the benzoic acid (Steps 7-9) to determine the freezing point of this mixture. When finished, put the test tube back in hot water, to melt the solution again. Using the proper test tube holder, take the test tube contents into the waste bottle. If there is any residue of the benzoic acid or lauric acid, you can rinse the test tube with 1-2ml of acetone. 13. When you have completed Step 9, you need to determine the temperature at which the mixture initially started to freeze. Unlike pure lauric acid, cooling a mixture of benzoic acid and lauric acid results in a gradual linear decrease in temperature during the time period when freezing takes place. In the Data screen (with the values x and y), locate the
Computer 15 15 - 4 Chemistry with Vernier initial freezing temperature of the solution. Record the freezing point in your notebook.
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