ColligativePropertiesLAB - Colligative Properties of...

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Colligative Properties of Solutions: Freezing Point Depression and Molar Mass Determination When a solute is dissolved in a solvent, the properties of the solvent are changed by the presence of the solute. The magnitude of the change generally is proportional to the amount of solute added. Some properties of the solvent are changed only by the number of solute particles present, without regard to the particular nature of the solute. Such properties are called Colligative properties of the solution. Colligative properties include changes in vapor pressure, boiling point, freezing point, and osmotic pressure. For example, if a nonvolatile, nonionizing solute is added to a volatile solvent, (such as water), the amount of solvent that can escape from the surface of the liquid at a given temperature is lowered, relative to the case where only the pure solvent is present. The vapor pressure above such a solution will be lower than the vapor pressure above a sample of the pure solvent under the same conditions. Molecules of nonvolatile solute physically block the surface of the solvent, thereby preventing as many molecules from evaporating at a given temperature. As shown in the figure, if the vapor pressure of the solution is lowered, there is an increase in the boiling point of the solution as well as a decrease in the freezing point. In this experiment, you will determine the freezing points of a pure solvent (naphthalene), a solution of a known solute (1,4-dichlorobenzene, C 6 H 4 C l2 ) dissolved in naphthalene, and an unknown solution of sulfur in naphthalene. The sulfur is considered to be an unknown solute because you will be determining the molar mass of sulfur (and the formula of elemental sulfur). It may turn out to be rather surprising. The decrease in freezing point ( T f ) when a nonvolatile, nonionizing solute is dissolved in a solvent is proportional to the molal concentration (m) of solute present in the solvent: T f = K f m K f is a constant for a given solvent (called the molal freezing point depression constant) and represents by how many degrees the freezing point will change when 1.00 mol of solute is dissolved per kilogram of solvent. For example, K f for water is 1.86oCkg/mol, whereas K f for the solvent benzene is 5.12oCkg/mol. The molal concentration of a solution represents how many moles of solute are
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This note was uploaded on 02/21/2011 for the course CH 201 taught by Professor Warren during the Spring '07 term at N.C. State.

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ColligativePropertiesLAB - Colligative Properties of...

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