Chapter 21

Colligative Properties:

Part 1

Four Colligative Properties of Solutions

Vapor pressure loweringVapor pressure loweringBoiling point elevation Boiling point elevation Freezing point depressionFreezing point depressionOsmotic pressureOsmotic pressure

Properties determined by the number of solute particles in solution rather than the type of particles.

What are the different properties of these solutions?

sugar and water salt and water

How do the colligative properties of these solutions compare?

sugar and water salt and water

Vapor Pressure Lowering

How Vapor Pressure Lowering Occurs

• Solute particles take up space in a solution.

• Solute particles on the surface decrease the number of solvent particles on the surface.

• Less solvent particles can evaporate from the surface which lowers the vapor pressure of the liquid.

Raoult’s LawVapor Pressure of solution =

(mole fractionsolvent)(Vapor Pressuresolvent)

Remember that the mole fractions in a solution must add up to equal “1”

Vapor Pressure Table

What is the vapor pressure of an aqueous solution What is the vapor pressure of an aqueous solution at 70at 70ooC if 2.00 x 10C if 2.00 x 1022g of sucrose (Cg of sucrose (C1212HH2222OO1111) are ) are

dissolved in 1.00 x 10dissolved in 1.00 x 1022g of water.g of water.

28.2 kPa

What is the vapor pressure of an aqueous solution What is the vapor pressure of an aqueous solution at 70at 70ooC if 2.00 x 10C if 2.00 x 1022g of sucrose (Cg of sucrose (C1212HH2222OO1111) are ) are

dissolved in 1.00 x 10dissolved in 1.00 x 1022g of water.g of water.

What is the vapor pressure of an aqueous solution What is the vapor pressure of an aqueous solution at 70at 70ooC if 3.15 x 10C if 3.15 x 1022g of Cg of C1515HH99NONO44 are dissolved in are dissolved in

5.00 x 105.00 x 1022g of water.g of water.

Answer: 29.9 kPa

What is the vapor pressure of an aqueous solution What is the vapor pressure of an aqueous solution at 70at 70ooC if 3.15 x 10C if 3.15 x 1022g of Cg of C1515HH99NONO44 are dissolved in are dissolved in

5.00 x 105.00 x 1022g of water.g of water.

29.9 kPa

Boiling Point Elevation

• The boiling point is the temperature at which the vapor pressure of a liquid equals the air pressure.

Boiling Point Elevation

• The boiling point is the temperature at which the vapor pressure of a liquid equals the air pressure.

• Dissolved solute particles lower the vapor pressure of the solvent. This requires a higher temperature to get the vapor pressure to equal air pressure.

Freezing Point Depression

• The freezing point is the temperature at which a liquid crystallizes to a solid.

• Solute particles disrupt the formation of crystals in the solvent.

• This disruption in the freezing process results in a lowering (depression) of the freezing point.

Homework

• Worksheet: Vapor Pressure, Boiling Point and Freezing Point. – You should be able to complete the front of this

worksheet – Due in two days

• Remember the “Review and Reinforcement” Worksheet is due tomorrow.