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Properties of Solutions(2)_CH17(Zum)-1-2

# Properties of Solutions(2)_CH17(Zum)-1-2 - COLLIGATIVE...

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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. Four important colligative properties are: vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. 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. This is discussed in the book in the experiment illustrated in fig 17.8. 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 (Fig 17.10). A solution that obeys Raoult’s law exactly is called an ideal solution. 1- What is the vapor pressure at 85 ° C above a 0.7500 m glucose solution? P 0 water at 85 ° C = 433.6 torrs. Glucose is a nonvolatile, nonionizing solute. 1

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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 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 Deviations from Raoult’s law are often observed.
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Properties of Solutions(2)_CH17(Zum)-1-2 - COLLIGATIVE...

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