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Chapter 11 Properties of Solutions Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Colligative Properties Depend only on the number, not on the identity, of the solute particles in an ideal solution: Boiling-point elevation Freezing-point depression Osmotic pressure Copyright © Cengage Learning. All rights reserved 2 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Boiling-Point Elevation Nonvolatile solute elevates the boiling point of the solvent. ΔT = Kbmsolute ΔT = boiling-point elevation Kb = molal boiling-point elevation constant msolute = molality of solute Copyright © Cengage Learning. All rights reserved 3 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Boiling Point Elevation: Liquid/Vapor Equilibrium To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 4 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Boiling Point Elevation: Addition of a Solute To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 5 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Boiling Point Elevation: Solution/Vapor Equilibrium To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 6 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Freezing-Point Depression When a solute is dissolved in a solvent, the freezing point of the solution is lower than that of the pure solvent. ΔT = Kfmsolute ΔT Kf msolute = freezing-point depression = molal freezing-point depression constant = molality of solute Copyright © Cengage Learning. All rights reserved 7 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Freezing Point Depression: Solid/Liquid Equilibrium To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 8 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Freezing Point Depression: Addition of a Solute To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 9 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Freezing Point Depression: Solid/Solution Equilibrium To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 10 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression Changes in Boiling Point and Freezing Point of Water Copyright © Cengage Learning. All rights reserved 11 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression EXERCISE! A solution was prepared by dissolving 25.00 g of glucose in 200.0 g water. The molar mass of glucose is 180.16 g/mol. What is the boiling point of the resulting solution (in °C)? Glucose is a molecular solid that is present as individual molecules in solution. Copyright © Cengage Learning. All rights reserved 12 Section 11.5 Boiling-Point Elevation and Freezing-Point Depression EXERCISE! A plant cell has a natural concentration of 0.25 m. You immerse it in an aqueous solution with a freezing point of –0.246°C. Will the cell explode, shrivel, or do nothing? Copyright © Cengage Learning. All rights reserved 13 Section 11.7 Colligative Properties of Electrolyte Solutions van’t Hoff Factor, i The relationship between the moles of solute dissolved and the moles of particles in solution is usually expressed as: moles of particles in solution i = moles of solute dissolved Copyright © Cengage Learning. All rights reserved 14 Section 11.7 Colligative Properties of Electrolyte Solutions Ion Pairing At a given instant a small percentage of the sodium and chloride ions are paired and thus count as a single particle. Copyright © Cengage Learning. All rights reserved 15 Section 11.7 Colligative Properties of Electrolyte Solutions Examples The expected value for i can be determined for a salt by noting the number of ions per formula unit (assuming complete dissociation and that ion pairing does not occur). NaCl i=2 KNO3 i=2 Na3PO4 i=4 Copyright © Cengage Learning. All rights reserved 16 Section 11.7 Colligative Properties of Electrolyte Solutions Ion Pairing Ion pairing is most important in concentrated solutions. As the solution becomes more dilute, the ions are farther apart and less ion pairing occurs. Ion pairing occurs to some extent in all electrolyte solutions. Ion pairing is most important for highly charged ions. Copyright © Cengage Learning. All rights reserved 17 Section 11.7 Colligative Properties of Electrolyte Solutions Modified Equations T = imK = iMRT Copyright © Cengage Learning. All rights reserved 18