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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 P1 = X1 P 10

Boiling-Point Elevation DTb = Kb m

Freezing-Point Depression DTf = Kf m

Osmotic Pressure (p) p = MRT

Lowering Vapor Pressure

Vapor pressure – pressure exerted by the vapor over the solution

Nonvolatile – no vapor pressure

P1 = X1 P 10Vapor-Pressure Lowering

(Raoult’s Law)

Glycerin (C3H8O3) 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.

P1 = X1 P 10Vapor-Pressure Lowering

P1 = new vapor pressure of solution X1 = mole fraction of solvent P1 = vapor pressure of pure solvent

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Pwater/glycerin solution = Xwater P pure water0

50.0 mL glycerin x 1.26 g x mol = 0.680 mol glycerin mL 92.11g500.0 mL H2O x 1g x mol = 27.75 mol H2O mL 18.02g

XH2O = 27.75 moL H2O = 0.9800 27.75 mol + 0.680 mol

Pwater/glycerin soln = 0.9800 (23.8 torr) Psolution = 23.3 torr

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PA = XA P A0

PB = XB P B0

PT = PA + PB

PT = XA P A0 + XB P B

0

Ideal Solution

When both the solute and solvent are volatile (have measureable vapor pressures) …

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PT is greater thanpredicted by Raoults’s law

PT is less thanpredicted by Raoults’s law

ForceA-B

ForceA-A

ForceB-B< & Force

A-BForceA-A

ForceB-B> &

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Osmotic Pressure (p)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 (p) is the pressure required to stop osmosis.

dilute moreconcentrated

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HighP

LowP

Osmotic Pressure (p)

p = iMRT

M is the molarity of the solutionR is the gas constantT is the temperature (in K)

solvent solution

time

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A cell in an:

isotonicsolution

hypotonicsolution

hypertonicsolution

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Chemistry In Action: Reverse Osmosis