Exp12 - Experiment 12 COLLIGATIVE PROPERTIES: FREEZING...

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Experiment 12 12—1 COLLIGATIVE PROPERTIES: FREEZING POINT DEPRESSION AND MOLAR MASS I. Learning Objectives… To become familiar with colligative properties and to use them to determine the molar mass of a substance. II. Background Information Solutions are homogeneous mixtures that contain two or more substances. The major component is called the solvent , and the minor component is called the solute . Since the solution is primarily composed of solvent, physical properties of a solution resemble those of the solvent. Some of these physical properties, called colligative properties , are independent of the nature of the solute and depend only upon the solute concentration. The colligative properties include vapor-pressure lowering, boiling-point elevation, freezing-point lowering, and osmotic pressure. The vapor pressure is just the escaping tendency of the solvent molecules. When the vapor pressure of a solvent is equal to atmospheric pressure, the solvent boils. At this temperature the gaseous and liquid states of the solvent are in dynamic equilibrium, and the rate of molecules going from the liquid state to the gaseous state is equal to the rate of molecules going from the gaseous state to the liquid state. It has been found experimentally that the dissolution of a nonvolatile solute (one with very low vapor pressure) in a solvent lowers the vapor pressure of the solvent, which in turn raises the boiling point and lowers the freezing point. This is shown graphically in Figure 1. You are probably familiar with some common uses of these effects: antifreeze is used to lower the freezing point and raise the boiling point of coolant (water) in an automobile radiator, and salt is used to melt ice. These effects are expressed quantitatively by the colligative-property law , which states that the freezing point and boiling point of a solution differ from those of the pure solvent by amounts that are directly proportional to
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12—2 Figure 1. Phase diagram for a pure solvent (black line) and a solution (blue line). the molal concentration of the solute. This relationship is expressed by the equation given below for the freezing-point lowering and boiling-point elevation Δ T=Km where T is the freezing-point lowering or boiling-point elevation, K is a constant specific for each solvent, and m is the molality of the solution (number of moles solute per kilogram solvent). Some representative constants, boiling points, and freezing points are given in Table 1. For cyclohexane, the solvent used in this experiment, the molal freezing- point depression constant (K fp ) has a value of 20.4 o C/m Table 1. Molal Freezing-Point and Boiling-Point Constants
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12—3 Solvent Freezing Point ( o C) K fp ( o C/m) Boiling Point ( o C) K bp ( o C/m) CH 3 COOH (acetic acid) 16.6 3.90 118.1 2.93 C 6 H 6 (benzene) 5.5 5.12 80.1 2.53 CHCl 3 (chloroform) -63.5 4.68 61.2 3.63 C 2 H 5 OH (ethanol) -114.6 1.99 78.4 1.22 H 2 O (water) 0.0 1.86 100.0 0.51 C 10 H 8 (naphthalene)
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This note was uploaded on 02/06/2010 for the course CHEM 102L taught by Professor N/a during the Spring '07 term at UNC.

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Exp12 - Experiment 12 COLLIGATIVE PROPERTIES: FREEZING...

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