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116 Chem Colligative Properties Colligative Properties Properties that depend on the number of solute particles dissolved in solution (amount), and not on the identity of the solute particle The FOUR Colligative properties are 1. 2. 3. 4. Note: Number of dissolved solute particles in solution, depend on whether the solute is a strong, weak, or non-electrolyte!!! Vapor Pressure Lowering CASE #1: Nonvolatile solute dissolved in solvent. Vapor Pressure: VP (pure solvent) > VP (solution) Chem 116 Colligative Properties Raoults Law Vapor pressure of a solution is dependent on how much (i.e. conc.) SOLVENT is present!! o Psoln = X solv Psolv Pso ln = vapor pressure of the solution X solvent = mole fraction of the solvent o Psolvent = vapor pressure of pure solvent Raoults Law works best for ideal solutions: 1. 2. Derive the expression for the change in Vapor Pressure. 1. Calculate the vapor pressure of a solution made by dissolving 20.0 g glucose, C6H12O6, in 500.0 g of water. The vapor pressure of pure water is 47.1 torr at 37C. 2. What is the vapor pressure (in mm Hg) of a solution made by dissolving 5.0 g of CaCl2 in 50.0 g of H2O at 70 C. The vapor pressure of pure water at 70 C is 233.7 mm Hg. 3. How many grams of sucrose must be added to 320. g of water to lower the vapor pressure by 1.5 mm Hg at 25 C? The vapor pressure of water at 25 C is 23.8 mm Hg, and the molar mass of sucrose is 342.3 g/mol. (4.1 x102 g) Chem 116 Colligative Properties Vapor Pressure Lowering CASE #2: Two (or more) volatile substances mixed. Both substances contribute to VP of solution. Psoln = P1 + P2 ..but, from Raoults Law Hence Sample Questions 1. In a solution of water in ethanol, the mole fraction of water is 0.25. If the vapor pressures of pure water and ethanol are 23.8 mm Hg and 61.2 mm Hg at 25 C, respectively, what is the vapor pressure of this solution? 2. Acetone, C3H6O, and ethyl acetate, C4H8O2, are organic liquids often used as solvents. At 30 the vapor pressure of pure acet one is 285 torr. What is the C, vapor pressure of pure ethyl acetate at 30 if th e vapor pressure of a solution of C, 25.0 g acetone in 25.0 g ethyl acetate is 219 torr? Chem 116 Colligative Properties Boiling Point Elevation Normal BP: The boiling point (BP) of a solution of a solvent with a nonvolatile solute is always HIGHER than the boiling point of the pure solvent. BPsoln > BPpure solvent TB,soln > TBo TB = TB,soln TBo Boiling Point Elevation What is the increase in the boiling point of the solution? this gives the CHANGE, not the new BP boiling point elevation T Tb = (Kb)(m) molality of solute particles molal BP elevation constant that is DIFFERENT for different solvents (units are Chem 116 Colligative Properties Freezing Point Depression Freezing point The temperature at which both the solid and liquid of the substance COEXIST The freezing point (FP) of a solution of a solvent with a nonvolatile solute is always LOWER than the freezing point of the pure solvent FPsoln < FPpure solvent TF,soln < ToF,solvent TF = ToF,solvent TF,soln Freezing Point Depression How much lower is the freezing point of the solution? this gives the CHANGE, not the new FP Tf = Kf m molality of solute particles # mols solute kg solvent molality = molal FP Depression constant that is freezing point depression DIFFERENT for different solvents (units are And different from Kb!! Chem 116 Colligative Properties Sample Questions 1. What are the boiling points of the following aqueous solutions? (for Kb water: = 0.51 C kg/mol) 0.90 m sugar solution 0.90 m Sr(ClO3)2 2. What is the normal boiling point (in C) of a solution prepared by dissolving 1.50 g of aspirin (acetylsalicylic acid, C9H8O4) in 75.00 g of chloroform (CHCl3)? The normal boiling point of chloroform is 61.7 C, and Kb for chloroform is3.63 3. What mass of NaCl is dissolved in 200. g of water if the freezing point of the solution is -2.14 C? Kf(water) = 1.86 4. A 0.100 g sample of PCB (a carcinogenic polychlorinated biphenyl) is dissolved in 10.0 g camphor. The freezing point of the resulting solution is 178.35 C. Calculate the molecular weight of the PCB. For camphor: Kf = 37.7 and Tf = 179.50 C 5. -carotene is a dietary source of vitamin A. Dissolving 0.0250 g of -carotene in 1.50 g of camphor gives a freezing-point depression of 1.17 C. What is the molar mass of -carotene? Kf for camphor is 37.7 (537 g/mol) 6. Which one of the following solutions has the highest boiling point? lowest boiling point? Kb(water) = 0.51 C kg/mol. 0.50 m sugar 0.50 m CaCl2 0.50 m NaCl 0.50 m HF 0.50 m AlCl3 7. Which one of the following solutions has the highest freezing point? lowest freezing point? 0.40 m Na2SO4 0.50 m KNO3 0.60 m C6H12O6 0.20 m Al(C2H3O2)3 0.60 m HC2H3O2 Chem 116 Colligative Properties Osmotic Pressure () semipermeable membrane: solvent molecules can pass through solute molecules cannot pass through Osmosis: Net movement of SOLVENT particles through a semipermeable membrane from a region of low concentration (dilute solution/pure solvent) to a region of high concentration. Osmotic pressure: The extra pressure required to stop osmosis of the solvent through the semipermeable membrane. Osmotic Pressure Experiments by vant Holf in 1887 showed that the osmotic pressure is related to the molarity of the solution according to the equation =MRT osmotic pressure (atm) molarity of the solution (mol/L) Kelvin temperature Ideal GAS CONSTANT (0.08206 L.atm/mol.k) This equation can also be re-written as (a form similar to the Ideal gas equation) Chem 116 Colligative Properties Osmotic Pressu...


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