1 Colligative Properties of Nonelectrolyte Solutions Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. Vapor-Pressure Lowering P 1 = X 1 P 1 0 Boiling-Point Elevation  T b = K b m Freezing-Point Depression  T f = K f m Osmotic Pressure (  )  = MRT

Lowering Vapor Pressure Vapor pressure – pressure exerted by the vapor over the solution Nonvolatile – no vapor pressure P 1 = X 1 P 1 0 Vapor-Pressure Lowering (Raoult’s Law)

Glycerin (C 3 H 8 O 3 ) is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25˚C. Calculate the vapor pressure at 25˚C of a solution made by adding 50.0mL of glycerin to 500.0mL of water. The vapor pressure of pure water at 25˚C is 23.8 torr. P 1 = X 1 P 1 0 Vapor-Pressure Lowering P 1 = new vapor pressure of solution X 1 = mole fraction of solvent P 1 = vapor pressure of pure solvent 0 P water/glycerin solution = X water P pure water mL glycerin x 1.26 g x mol = mol glycerin mL 92.11g mL H 2 O x 1g x mol = mol H 2 O mL 18.02g X H2O = moL H 2 O = mol mol P water/glycerin soln = (23.8 torr) P solution = 23.3 torr

4 P A = X A P A 0 P B = X B P B 0 P T = P A + P B P T = X A P A 0 + X B P B 0 Ideal Solution When both the solute and solvent are volatile (have measureable vapor pressures) …

5 P T is greater than predicted by Raoults’s law P T is less than predicted by Raoults’s law Force A-B Force A-A Force B-B <& Force A-B Force A-A Force B-B >&

6 Osmotic Pressure (  ) Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one. A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules. Osmotic pressure (  ) is the pressure required to stop osmosis. dilute more concentrated

7 High P Low P Osmotic Pressure (  )  = iMRT M is the molarity of the solution R is the gas constant T is the temperature (in K) solvent solution time

8 A cell in an: isotonic solution hypotonic solution hypertonic solution

9 Chemistry In Action: Reverse Osmosis