Colligative

Properties

Vapor Pressure Lowering

The bonds between molecules keep molecules of a liquid from escaping into the vapor state.

In a solution, some of the solvent is busy keeping the solute dissolved.

This lowers the vapor pressure.

Vapor Pressure Lowering

Electrolytes form ions when dissolved.

NaCl Na+ + Cl- 2 ionsCaCl2 Ca+2 + 2Cl- 3 ions Calcium chloride would lower the

vapor pressure more because it dissociates into more ions.

Boiling Point Elevation

The vapor pressure determines the boiling point.

The lower the vapor pressure, the higher boiling point.

It turns out that the boiling point of a solution is higher than the boiling point of the pure solvent.

Boiling Point Elevation

Salt water boils above 100ºC.KBr K+ + Br- 2 ionsMgF2 Mg+2 + 2F- 3 ions Magnesium fluoride would raise the

boiling point more because it dissociates into more ions.

Freezing Point Depression

Solids form when molecules make an orderly pattern.

The solute molecules break up the orderly pattern.

This makes the freezing point lower.

Freezing Point Depression

Salt water freezes below 0ºC.K2SO4 2K+ + SO4

-2 3 ionsNaCl Na+1 + Cl- 2 ions Potassium sulfate would lower the

freezing point more because it dissociates into more ions.

MolalityMolality is another unit for

concentration.

m =

1 kg = 1000 g

moles of solute

kilogram of solvent

Example

What is the molality of a solution with 9.3 mole of NaCl in 450 g of water?

moles of solute

kilogram of solventm =

0.450 kg

9.3 mol

m = 21 m

Problem

What is the molality of a solution with 11.3 mole of KBr in 650. g of water?

(17.4 m)

Problem

What is the molality of a solution with 142 g of KBr in 750. g of water?

(1.59 m)

Why Molality?

The size of the change in boiling point is determined by the molality.

Tb = Kb x m x nTb is the change in the boiling pointKb is a constant determined by the

solventm is the molality of the solution.n is the number of ions the solute

falls into when it dissolves.

What about Freezing?The size of the change in freezing

point is determined by the molality.Tf = -Kf x m x nTf is the change in the boiling pointKf is a constant determined by the

solvent.m is the molality of the solution.n is the number of ions the solute

falls into when it dissolves.

Example What is the boiling point of a solution

made by dissolving 1.20 moles of NaCl in 750. g of water? Kb for water is 0.51.

moles of solute

kilogram of solventm =

0.750 kg

1.20 mol

m = 1.60 m

Example What is the boiling point of a solution

made by dissolving 1.20 moles of NaCl in 750. g of water? Kb for water is 0.51.

KbmΔTb = (1.60)0.51

ΔTb = 1.63 °C

n(2)

So Tb = 100 + 1.63 = 101.63 °C

Example What is the freezing point of a solution

made by dissolving 1.45 moles of CaCl2 in 850. g of water? Kf for water is 1.86.

moles of solute

kilogram of solventm =

0.850 kg

1.45 mol

m = 1.71 m

Example What is the freezing point of a solution

made by dissolving 1.45 moles of CaCl2 in 850. g of water? Kf for water is 1.86.

- KfmΔTf = (1.71)1.86

ΔTf = - 9.54 °C

n(3)

So Tf = 0 + - 9.54 = - 9.54 °C

ColloidsSometimes, mixtures are partway

between true solutions and heterogeneous mixtures.

Such mixtures, called colloids, contain particles that are evenly distributed through a dispersing medium, and remain distributed over time rather than settling out.

ColloidsFamiliar colloids include fog, smoke,

homogenized milk, and ruby-colored glass.

ColloidsA beam of light can be used to

distinguish a colloidal suspension from a true solution. This is referred to as the Tyndall Effect.

The Tyndall Effect is caused by reflection of light by very small particles in suspension in a trasnparent medium.

ColloidsA beam of light can be used to

distinguish a colloidal suspension from a true solution. This is referred to as the Tyndall Effect.

The Tyndall Effect is caused by reflection of light by very small particles in suspension in a trasnparent medium.

ColloidsThe Tyndall effect is seen below

using a laser pointer. The glass on the left contains colloidal silver and the one on the right is water from the tap after the bubbles have settled out.