cp19 - Colligative Properties: Freezing-Point Depression...

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1 Colligative Properties: Freezing-Point Depression and Molar Mass Names : Andrew Yaksic , Doug Gately Purpose : To become familiar with colligative properties and to use them to determine the molar mass of a substance. Equipment : 600-mL beaker, thermometer, large test tube, 250-mL wide-mouth glass bottle, paper towels, wire gauze, clamp, standard laboratory balance, analytical balance, Bunsen burner, rubber hose, wire stirrer, weighing paper, ring stand, iron ring, two-hole rubber stopper with slit Materials : sulfur, naphthalene Introduction : 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, measured in molality , or moles of solute per kilogram of solvent. The colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. The vapor pressure is the escaping tendency of 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 to the gaseous state is equal to the rate of molecules going from the gaseous state to the liquid state. The phase diagram below illustrates the effect of adding a solute to a pure substance.
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2 As is demonstrated by the phase diagram above, adding a solute to a solvent lowers the freezing point and raises the boiling point; it also lowers the vapor pressure. The new freezing point of a solution can be determined using the colligative property law: T f = k f m The change in freezing point is equal to the molal freezing-point constant times the molality of the solution. The molal freezing-point constant used is the constant for the solvent, not the solute. In this experiment, the molar mass of sulfur will be determined using the colligative property law. The freezing point of naphthalene will be determined experimentally; then a controlled solution of naphthalene and sulfur will be made, and the freezing point of that solution will be determined. The difference in freezing point can be used in the colligative property law to determine the experimental molality of the solution, leading to a calculation of molecular weight.
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3 The freezing temperature is difficult to ascertain by direct visual observation because of a phenomenon called supercooling and also because solidification of solutions usually occurs over a broad temperature range. Temperature-time graphs, called cooling curves , reveal freezing temperatures rather clearly. The cooling curve will look like the one below in figure 19.2:
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This note was uploaded on 12/09/2011 for the course SP 108 taught by Professor Whittenburg during the Summer '11 term at Montgomery College.

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cp19 - Colligative Properties: Freezing-Point Depression...

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