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# Colligative properties

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colligative properties, solutions, vapor pressure, boiling and freezing point changes, osmotic pressure

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### Colligative properties

1. 1. Solutions Colligative Properties • Changes in colligative properties depend only on the number of solute particles present, not on the identity of the solute particles. • Among colligative properties are Vapor pressure lowering Boiling point elevation Melting point depression Osmotic Pressure
2. 2. Solutions Vapor Pressure Because of solute- solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase.
3. 3. Solutions Vapor Pressure Therefore, the vapor pressure of a solution is lower than that of the pure solvent.
5. 5. Solutions Boiling Point Elevation and Freezing Point Depression Nonvolatile solute- solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.
6. 6. Solutions Boiling Point Elevation The change in boiling point is proportional to the molality of the solution: Tb = Kb  m where Kb is the molal boiling point elevation constant, a property of the solvent.Tb is added to the normal boiling point of the solvent.
7. 7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Sample Problem E What is the boiling-point elevation of a solution made from 20.1 g of a nonelectrolyte solute and 400.0 g of water? The molar mass of the solute is 62.0 g. Chapter 13 Section 2 Colligative Properties of Solutions Boiling-Point Elevation, continued
8. 8. Solutions Freezing Point Depression • The change in freezing point can be found similarly: Tf = Kf  m • Here Kf is the molal freezing point depression constant of the solvent. Tf is subtracted from the normal freezing point of the solvent.
9. 9. Solutions Boiling Point Elevation and Freezing Point Depression Note that in both equations, T does not depend on what the solute is, but only on how many particles are dissolved. Tb = Kb  m Tf = Kf  m
10. 10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Sample Problem C What is the freezing-point depression of water in a solution of 17.1 g of sucrose, C12H22O11, in 200. g of water? What is the actual freezing point of the solution? Chapter 13 Section 2 Colligative Properties of Solutions Freezing-Point Depression, continued
11. 11. Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Chemistry: The Central Science, Eleventh Edition By Theodore E. Brown, H. Eugene LeMay, Bruce E. Bursten, and Catherine J. Murphy With contributions from Patrick Woodward Sample Exercise 13.9 Calculation of Boiling-Point Elevation and Freezing-Point Lowering Calculate the freezing point of a solution containing 0.600 kg of CHCl3 and 42.0 g of eucalyptol (C10H18O), a fragrant substance found in the leaves of eucalyptus trees. Practice Exercise
12. 12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Osmotic Pressure • A semipermeable membrane allows the passage of some particles while blocking the passage of others. • The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration is osmosis. • Osmotic pressure is the external pressure that must be applied to stop osmosis. Chapter 13 Section 2 Colligative Properties of Solutions