Experiment 1: Determination of a Molar Mass by Freezing Point Depression Chloe Alberts 1002129519 PRA127 February 11th, 2016 February 25th, 2016 Introduction:
Freezing point depression is the phenomena that describes why adding a solute to a solvent results in the lowering of the freezing point of the solvent. When a substance starts to freeze, the molecules slow down due to the decreases in temperature, and the intermolecular forces start to take over. The molecules will then arrange themselves in a pattern, and thus turn into a solid. 1 The purpose of this experiment was to determine the molar mass of an unknown, non- volatile solute. To determine this, the measurement of temperature as a function of time was used as the liquid solvent was cooled to its freezing point, and cooling curves were made to measure the solute’s effect on the freezing point of the liquid solvent. 2 Procedure: In this experiment, an ice-water bath was prepared by adding 300 mL of ice and 100 mL of water into a 400 mL and then NaCl was added into the beaker. To ensure the temperature stayed between -5 to -10 o C, the ice-water bath was placed into a 600 mL beaker for insulation. The mass of a dry test tube in a 100ml beaker was then found and was weighed to be 65.68g. 12ml of cyclohexane was then added to this test-tube and the new weight of the test tube and beaker was weighed to be 74.89g. The test tube was put into the 400ml beaker, and clamped in place, then a thermometer was secured in place inside the test tube with a stirring rod. The temperature of the solution was then recorded every 15 seconds for 7 minutes. 0.128g of unknown solute #1 was measured and placed in the test-tube, then the temperature was recorded again in 15 second intervals for 7 minutes. This was repeated two more times, adding 0.115g of solute first, and 0.118g for the last procedure 2 .
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