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UWW CHEM 104 - Raoult's Law, and Vapor Pressure Lowering

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Chem 104 1st Edition Lecture 17Outline of Last Lecture I. Solution Concentration and mole fractionsA. Remarks concerning mole fractionsB. Example of how to solve for mole fractionsII. Colligative propertiesIII. Vapor pressure and Raoult’s lawA. Solving for a vapor pressure by using Raoult’s lawOutline of Current Lecture I. Continued practice with Raoult’s law.A. Example problems to work with concerning Raoult’s lawII. Vapor Pressure Lowering and its principles and equationIII. Raoult’s law for a volatile soluteIV. Example problem due for extra credit.Current LectureRaoult’s LawI. Example 1: Calculate the total vapor pressure of a solution made by dissolving 25.0 g of glucose (C6H12O6) in 215 g of water at 50°CGiven: 25.0 g C6H12O6, 215 g H2O, P°= 92.5 torr, MM C6H12O6= 180.2 g, MM H2O = 18.02gFind: PH2OPsolvent∈solution=Xsolvent∗P °First convert g to mol in both H2O and C6H12O6:1mol180.2 g=¿¿25.0 g ¿215 g(1 mol18.02 g)=11.93 molNow solve for mole fraction:XH 2O=11.93 mol(0.1387 mol+11.93 mol)=0.9885Finally solve for PH2O:PH 2 O=(0.9885) (92.5torr)=91.4 torrVapor Pressure LoweringI. An easy way of viewing vapor pressure lowering is by putting it in terms of what it is equal and not equal to:A. Vapor Pressure Solvent in solution ˂ Vapor pressure of pure solventB. Vapor pressure of solution = amount of solvent in solutionC. Vapor pressure of pure solvent – vapor pressure in solute = vapor pressure lowering.D. It is represented in the equation:∆ P=P °solvent−Psolution= Xsolute∗P° solventNotice that the last part of the equation is Raoult’s law. Another way of viewing Raoult’s law:Xsolute∗P °=(1−Xsolvent)P °solventRaoult’s Law for volatile soluteI. When both the solvent and the solute can evaporate, both molecules will be found in the vapor phase:Ptotal=Psolute+PsolventYou can apply Raoult’s law because their volatile. The solute vapor pressure decreases in the same way the solute decreases the solvents:Psolute=Xsolute∗P °Psolvent= Xsolvent∗P °solventII. This problem needs to be solved and turned into class for extra credit:A. Calculate the component and total vapor pressure of a solution prepared by mixing 3.95 g of CS2 with 2.43 g of C3H6O.Given: 3.95 g CS2, 2.43 g C3H6O, P°CS2=515 torr, P°= 332 torrFind: PCS2, PC3H6O,


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UWW CHEM 104 - Raoult's Law, and Vapor Pressure Lowering

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