# Molar Mass Determination By Freezing Point Depress

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Experiment 15: Molar Mass Determination by Freezing Point Depression Introduction The purpose of this experiment is to determine the molar mass of camphor by determining the freezing point depression of cyclohexane. In this experiment a sample of the solvent cyclohexane was cooled to its freezing point and a cooling curve was constructed. A known mass of camphor was added to the known mass of camphor and the freezing point of the solution was determined using equations 1 and 2. From the freezing point depression the molality of the camphor can be calculated and then the molar mass, by using equation 3. If Tf(solvent) is defined as the freezing point of the pure solvent, and if Tf(solution) is defined as the freezing point of the solution, then the freezing point depression (in °C) is given by equation 1: ΔTf = Tf(solvent) - Tf(solution) (1) The freezing point depression ΔTf is related to m, the molality of the solution particles, given in equation 2: ΔTf = Kf m (2) If a known mass of a solute is placed into a known mass of solvent, and the freezing point depression relative to the pure solvent determined, then the molar mass (MM) of the solute can be determined by using the appropriate Kf value for the solvent, equation 3: ΔTf = Kf (mol/kg solvent) = Kf (g solute/MM solute) / kg solvent (3) Procedure Determination of the Freezing Point of Cyclohexane A 20 x 50 mm test tube was rinsed clean with cyclohexane. Then the test tube, stopper, and a 150 ml beaker were weighed on a top loading balance. Approximately 4.9 grams of cyclohexane was added to the test tube then the tube, stopper, and beaker was reweighed. A salt-ice bath was prepared with approximately 45 g of rock salt added to a 1 L