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UA CH 102 - Colligative Properties of Solutions

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CH 102 1st Edition Lecture 7 Outline of Current Lecture I. Temperature Dependence of Solubility of Gases in Water II. Henry’s LawIII. Vapor Pressure of SolutionsIV. Vapor Pressure LoweringV. Raoult’s Law and Vapor PressureVI. Raoult’s Law for Volatile SoluteVII. Freezing Point DepressionCurrent LectureI. Temperature Dependence of Solubility of Gases in Water a. Gases generally have lower solubility in water than ionic or polar covalent solids because most are nonpolar molecules.b. Usually, gases with high solubility are actually reacting with water.c. For all gases, the solubility of the gas decreases as the temperature increases.d. The ΔHsolution is exothermic because you do not need to overcome solute–solute attractions.II. Henry’s Lawa. The solubility of a gas (Sgas) is directly proportional to its partial pressure, (Pgas).b. Sgas = kHPgasc. kH is called the Henry’s law constant.d.III. Vapor Pressure of Solutionsa. The vapor pressure of a solvent above a solution is lower than the vapor pressureof the pure solvent.i. The solute particles replace some of the solvent molecules at the surface.ii. The pure solvent establishes a liquid vapor equilibrium.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.iii. Addition of a nonvolatile solute reduces the rate of vaporization, decreasing the amount of vapor.iv. Eventually, equilibrium is re-established, but with a smaller number of vapor molecules; therefore, the vapor pressure will be lower.b. A concentrated solution will draw solvent molecules toward it because of the natural drive for materials in nature to mix.i. Similarly, a concentrated solution will draw pure solvent vapor into it because of this tendency to mix.ii. The result is reduction in vapor pressure.IV. Vapor Pressure Loweringa. The vapor pressure of a solvent in a solution is always lower than the vapor pressure of the pure solvent.b. The vapor pressure of the solution is directly proportional to the amount of the solvent in the solution.c. The difference between the vapor pressure of the pure solvent and the vapor pressure of the solvent in solution is called the vapor pressure lowering.i. ΔP = P°solvent – Psolution = χsolute P∙ °solventV. Raoult’s Law and Vapor Pressurea. The vapor pressure of a volatile solvent above a solution is equal to its normal vapor pressure, P°, multiplied by its mole fraction in the solution.i. Psolvent in solution = χsolvent ∙ P°b. Because the mole fraction is always less than 1, the vapor pressure of the solventin solution will always be less than the vapor pressure of the pure solvent.VI. Raoult’s Law for Volatile Solutea. When both the solvent and the solute can evaporate, both molecules will be found in the vapor phase.b. The total vapor pressure above the solution will be the sum of the vapor pressures of the solute and solvent.1. For an ideal solution,ii. Ptotal = Psolute + Psolventc. The solvent decreases the solute vapor pressure in the same way the solute decreased the solvent’s.i. Psolute = χsolute P∙ °solute and Psolvent = χsolvent P∙ °solvent 1. The mole fractions in the vapor phase are not necessarily the same as the mole fractions in solutionVII. Freezing Point Depressiona. The freezing point of a solution is lower than the freezing point of the pure solvent.i. Therefore, the melting point of the solid solution is lower.b. The difference between the freezing point of the solution and freezing point of the pure solvent is directly proportional to the molal concentration of solute particles.i. (FPsolvent – FPsolution) = ΔTf = m ∙ kf1. The proportionality constant is called the freezing point depression constant, kf.2. The value of kf depends on the solvent.3. The units of kf are

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