Molar Mass Essay

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Determination of Molar Mass by Freezing-Point Depression When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the number of moles of solute added. This property, known as freezing-point depression, is a colligative property; that is, it depends on the ratio of solute and solvent particles, not on the nature of the substance itself. The equation that shows this relationship is ΔTf = Kf × m× i where ΔTf is the freezing point depression, Kf is the freezing point depression constant for a particular solvent (3.9°C•kg/mol for lauric acid in this experiment1), i is the van’t Hoff factor, and m is the molality of the solution (in mol solute/kg solvent). Since lauric acid is not ionic, its van’t Hoff factor is essentially equal to 1. OBJECTIVES In this experiment, you will • • • • Determine the freezing temperature of the pure solvent, lauric acid. Determine the freezing temperature of a mixture of lauric acid and benzoic acid. Calculate the freezing point depression of the mixture. Calculate the molecular weight of benzoic acid. Figure 1 MATERIALS Data Collection Mechanism Temperature Probe ring stand 400 mL beaker Tissue or paper towels lauric acid, CH3(CH2)10COOH lauric acid-benzoic acid mixture hot water bath utility clamp two 18 × 150 mm test tubes (if pre-made samples are not provided by your teacher) 1 “The Computer-Based Laboratory”, Journal of Chemical Education: Software, 1988, Vol.1A, No. 2, p. 73. Adapted from Advanced Chemistry with Vernier & Laboratory Experiments for Advanced Placement Chemistry by Sally Ann Vonderbrink, Ph. D. The Determination of Molar Mass by Freezing-Point Depression PROCEDURE 1. Obtain and wear goggles. 2. Set up the data collection system. a. Connect a Temperature Probe to the interface. b. Start the data collection program. c. Set up the time graph for 10 seconds per sample and 60
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