Properties of Solutions_part_2_S13

# Properties of Solutions_part_2_S13 - COLLIGATIVE PROPERTIES...

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COLLIGATIVE PROPERTIES OF SOLUTIONS These are properties that depend on the number and not the kind of particles in a given amount of solvent. Vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure are all colligative properties. THE VAPOR PRESSURE OF SOLUTIONS The properties of solutions are significantly different than the properties of pure solvents. One property that is affected is the vapor pressure. Aqueous solution and pure water in a closed environment Zumdahl, S. S. Chemical Principles , 6 th ed.; Boston: Houghton Mifflin Company, 2010. A solution containing a non-volatile solute has a vapor pressure that is lower than the vapor pressure of the pure solvent. Francois Raoult conducted studies on the vapor pressure of solutions that contain non-volatile solute and concluded the following (known as Raoult’s Law): “The vapor pressure of a solvent in an ideal solution is directly proportional to the mole fraction of the solvent in the solution.” Raoult’s law for a solution containing a non-volatile solute is described in the following equation: P soln = X solvent P 0 solvent X is the mole fraction of the solvent P 0 solvent is the vapor pressure of the pure solvent Note that Raoult’s law is a linear equation. 1

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Plot of solution that obeys Raoult's Law Zumdahl, S. S. Chemical Principles , 6 th ed.; Boston: Houghton Mifflin Company, 2010. A solution that obeys Raoult’s law exactly is called an ideal solution. 1- What is the vapor pressure at 25 ° C above a 2.50 m glucose solution? P 0 water at 25 ° C = 23.8 torr. What happens when the solute is volatile? When a solution contains two volatile compounds, A and B, each component exerts its own partial vapor pressure above the surface of the solution: P A = X A P 0 A P B = X B P 0 B The total vapor pressure of the solution ( P total ) is a modified form of Raoult’s law: P total = P A +P B or P total = X A P 0 A + X B P 0 2
A solution that obeys this modified form of Raoult’s law exactly is called an ideal solution. Example: At 25.0 ° C, the vapor pressure of benzene is 96.0 mm Hg and that of toluene is 30.3 mm Hg. If 78.0 g of benzene (78.1 g/mol) are mixed with 92.0 g of toluene (92.1 g/mol), what is the vapor pressure of the solution? Calculate the mole fraction of benzene and toluene that is in equilibrium with the solution. Non-Ideal Solutions Vapor pressure for a solution of two volatile liquids: (a) ideal behavior, (b) positive deviation, (c) negative deviation Zumdahl, S. S. Chemical Principles , 6 th ed.; Boston: Houghton Mifflin Company, 2010.

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