apparatus with the lauric acid and unknown. We clamped the apparatus in the water bath and melted the mixture of the two solids. We stirred the completely melted solution for several minutes to ensure adequate mixing to form a homogeneous solution. We then continued with the cooling procedure by removing the test tube from the hot water bath
and recorded the temperature to the nearest 0.2°C every 30 seconds under constant stirring. We cooled the apparatus in the air for about 5 minutes and then in the cold water bath for an additional 5 minutes. We then repeated experiment one, by cleaning our apparatus and measuring out a new amount of lauric acid again of about 6.3 g. this was the second experiment and second trail of experiment one. After experiment two was completed we added 1.5 grams of the specified unknown, and repeated the steps we took in experiment three. This was the last experiment, experiment 4, at the completion of the experiment, we melted the lauric acid solution and poured it into the waste container. We thoroughly cleaned the glassware, stir wire, and thermometer by scrubbing it with a brush while washing it with soap and hot water in the sink. Results A. Preparation of experimental apparatus and freezing point determination of lauric acid Exp 1 Exp 2 Mass of freezing point depression apparatus 20.848 g 47.089 g Mass of freezing point depression apparatus w/ lauric acid 27.177 g 53.4 g Mass of lauric acid used 6.329 g 6.311 g B. Freezing point determination of a lauric acid solution Designation of unknown Benzoic Acid Mass of added unknown, Exp 3 1.05 g Mass of added unknown, Exp 4 1.505 g Freezing point of lauric acid from Part A, Exp 1 31.2 °C Freezing point of lauric acid from Part A, Exp 2 31.8 °C Average freezing point of lauric acid 31.5 °C Freezing point of lauric acid and unknown from Part B, Exp 3 28.1 °C Freezing point of lauric acid and unknown from Part B, Exp 4 27.5 °C Delta T f = T f, mixture – T f, pure Exp.3 -3.1 °C Exp.4 -4.3 °C Solution molality Exp.3 0.79 m Exp.4 1.103 m Moles of unknown Exp.3 4.98x10^-3 mol Exp.4 6.95x10^-3 mol Molar mass of unknown Exp.3 210.84 g/mol Exp.4 216.55 g/mol Average molar mass 213.695 g/mol
Discussion Our freezing point depression experiments were used to determine the molar mass of our unknown substance. The solution we prepared by dissolving 1 gram in the 3
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- Fall '09
- Chemistry, Vapor pressure, Freezing-point depression