Objective/Warm-Up

SWBAT define and identify colligative properties.

What are some differences between molarity and molality?

Teams

6 Team leaders 4-5 people in a team I will grade one paper from each team-

the blue or goldenrod colored paper. All team members must have their

paper filled out before I will collect the team paper.

Use pp. 446-457 in Modern Chemistry

Use pp. 446-457 in Modern Chemistry

Molality

Molality: m =moles solute/kg solvent

How is molality different than molarity?

How would these differences affect how to make a solution based on molality as opposed to molarity?

• To make a 0.500m solution of NaCl, use a balance to measure 1.000 kg of water and add 0.500 mol (29.3 g) of NaCl.

Objective/Warm-Up SWBAT define and identify colligative

properties. A 4 g sugar cube (Sucrose: C12H22O11)

is dissolved in a 341 g of water. What is the molality of the sugar solution?

molality = molsolute / masssolvent

molality = 0.0117 mol / 0.341 kg

molality = 0.034 mol/kg

C12H22O11 = 342 g/mol

4 g /(342 g/mol) = 0.0117 mol

Objective/Warm-Up

SWBAT describe colligative properties.

Why do we use salt to “melt” the ice on the road? How does salt “melt” the ice on the road?

Does adding a salt to water make the freezing point go up or down?

Does adding a salt to water make the boiling point go up or down?

Colligative Properties of Solutions

The wood frog is a remarkable creature because it can survive being frozen. Scientists believe that a substance in the cells of this frog acts as a natural antifreeze, which prevents the cells from freezing. You will discover how a solute can change the freezing point of a solution.

Colligative Properties

Properties that depend on the amount of particles in solution, not the identity of the solute.

Four important colligative properties of solutions are• vapor-pressure lowering• boiling-point elevation• freezing-point depression• Osmotic pressure

Colligative Properties

Reduction in Vapor Pressure

Less surface area Solvent molecules

attracted to solute particles

Vapor pressure lowering--The decrease in a solution’s vapor pressure is proportional to the number of particles the solute makes in solution.

Colligative Properties

Freezing point depression:• T = Kf d.f. m

Boiling point elevation:• T = Kb d.f. m

Change in temperature constant Dissociation factor molality

Dissociation Factor: a) Covalent compounds: DF=1b) Ionic compounds: DF= # of ions in compound

DF = 2DF = 1

c) CaCl2

DF = 3

The freezing-point depression of aqueous solutions makes walks and driveways safer when people sprinkle salt on icy surfaces to make ice melt. The melted ice forms a solution with a lower freezing point than that of pure water.

Freezing-Point Depression•The difference in temperature between the freezing point of a solution and the freezing point of the pure solvent is the freezing-point depression.

Boiling-Point Elevation• The difference in temperature between the

boiling point of a solution and the boiling point of the pure solvent is the boiling-point elevation.

• The same antifreeze added to automobile engines to prevent freeze-ups in winter, protects the engine from boiling over in summer.

Simulation

http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/propOfSoln/colligative.html

Osmotic Pressure The pressure required to prevent

osmosis

Desalination by Reverse Osmosis

1.Which of the following is NOT a colligative property of

a) vapor-pressure loweringb) freezing-point depressionc) boiling-point elevationd) solubility elevation

16.3 Review

2. Choose the correct word for the space: The magnitude of each colligative property of solutions is proportional to the __________ solute dissolved in the solution.

a) type ofb) number of particles ofc) molar volume ofd) particle size of the

3. The decrease in vapor pressure when a solute is added to a liquid is due to

a) attractive forces between solvent particles.b) repulsion of the solute particles by the solvent

particles.c) dissociation of the solvent particles.d) attractive forces between solvent and solute

particles.

4.You have 500 mL of 1M solutions of NaCl, Na2SO4, Na3PO4, and Al2(SO4)3. Which solution will have the highest boiling point?

a) NaCl(aq)b) Na2SO4(aq)

c) Na3PO4(aq)

d) Al2(SO4)3(aq)

WHY?