# Chapter17c - CHAPTER 17b Solutions 1 Lowering of Vapor...

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1 CHAPTER 17b Solutions 2 Lowering of Vapor Pressure and Raoult’s Law ± To determine how a nonvolatile solute affects a solvent, we can consider what occurs when a beaker containing an aqueous sugar solution and a beaker of water are sealed under a container. 1. We will notice that the volume of the aqueous sugar solution is increasing while the volume of the pure water is decreasing over time. 2. The change is brought about because the amount of vapor equilibrium for the two substances is different. 3 Figure 17.8: An aqueous solution and pure water in a closed environment The vapor pressure needed for the water to reach equilibrium in the container is greater for the water than it is for the solution. As the water vapor forms, it is absorbed by the solution.

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4 Lowering of Vapor Pressure and Raoult’s Law pure water vapor → ← ± As the vapor is formed, it is absorbed by the solution. The vapor equilibrium requires that the water produce more vapor to reestablish it equilibrium in the container. ± As the vapor pressure increases in the container, the solution absorbs the excess vapor to reestablish its vapor pressure equilibrium solution vapor 5 Lowering of Vapor Pressure and Raoult’s Law ± Addition of a nonvolatile solute to a solution lowers the vapor pressure of the solution. ² The effect is simply due to fewer solvent molecules at the solution’s surface. ² The solute molecules occupy some of the spaces that would normally be occupied by solvent. ² In a solution consisting of half solute and half solvent molecules, the observed vapor pressure of the solution is half that of the pure solvent vapor pressure. 6 Figure 17.9: The presence of a nonvolatile solute inhibits the escape of solvent molecules from the liquid
7 Lowering of Vapor Pressure and Raoult’s Law ± Raoult’s Law models this effect in ideal solutions. ± Derivation of Raoult’s Law. Determining the vapor pressure of a solvent in solution: 0 solution solvent solvent soln 0 solvent solvent PP where P vapor pressure of P vapor pressure of pure solvent mole fraction of solvent X the solution X in solution = = = = 8 Lowering of Vapor Pressure and Raoult’s Law ± This graph shows how the solution’s vapor pressure is changed by the mole fraction of the solute, which is Raoult’s law. Vapor pressure of pure solvent. 9 Ideal Solutions ± Any solution that obeys the Raoult’s Law is called an ideal solution. ² Ideal behavior is very closely observed when the solute- solute, solvent-solvent, and solute-solvent interactions are very similar. ± When the solute and solvent are alike, the solute acts to dilute the solvent when it is added. ² A solution behaves ideal when the H soln = 0. When the solute and solvent are very alike.

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10 Lowering of Vapor Pressure and Raoult’s Law ± Example: Sucrose is a nonvolatile, nonionizing solute in water. Determine the vapor pressure lowering, at 25.0 o C, of a 1.25 m sucrose solution. Assume the solution behaves ideally.
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## Chapter17c - CHAPTER 17b Solutions 1 Lowering of Vapor...

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