outline for today - colby collegeweb.colby.edu/ch141/files/2018/12/lecture-slides-12-5.pdf · 2018....

Outline for Today Wednesday, Dec. 5

• Chapter 11: Intermolecular Forces and Liquids

• Phase Changes

• Chapter 13: Properties of Solutions

• Solubility of Gases

• Colligiative Properties

!1

Example Problem

• Why is methanol (CH3OH) miscible in water but hexanol (CH3CH2CH2CH2CH2CH2OH) is not?

H3C

H2C

CH2

H2C

CH2

H2C

OHC OHH

HH

OH H

hexanolmethanol

water

Saturated Solutions

• Saturated solution: A solution that is at equilibrium with undissolved solute.

Additional solute will not dissolve.

solute + solvent solution

• The rate of dissolving is equal to the rate of crystalizing.

dissolve

crystalize

Supersaturated Solutions

Example Problem: Saturated solutions

Using the graph, will the addition of 40.0 g of potassium chloride to 100.0 g of water at 80oC lead to a saturated solution or unsaturated solution?

Solubility of gases• Henry’s Law Solubility of a gas increases directly with

increasing pressure

More collisions with the surface of the liquid increases dissolving.

Sg=kPg

Temperature Effects• Solubility of (most) solids increases with increasing temperature.

• Solubility of gases decreases with increasing temperature.

• Why?

• The best answer involves entropy!

• Higher the temperature, the higher the kinetic energy of all substances.

• The higher the kinetic energy, the more likely for the dissolved gas to escape the solution and become a gas.

Let’s try it!

Units of Concentration

• Molarity (M) =

• Molality (m) =

• Mole Fraction (X) =

Moles of Solute

Liters of Solution

Moles of Solute

Kilograms of Solvent

Moles of Solute

Total Moles

Units of Concentration

• Mass Percent =

• parts per million (ppm) =

• parts per billion (ppb) =

mass of solute

total mass of solution

mass of solute

total mass of solution

x 100

x 106

mass of solute

total mass of solutionx 109

Colligative Properties Depend on the Concentration (not identity) of Solute

Particles

• The lowering of Vapor Pressure depends on the mole fraction of the solvent.

• The increase of Osmotic Pressure increase depends on the molarity (M) of the ions.

• Freezing Point Depression depends on molality (m)

• Boiling Point Elevation depends on molality (m).

Vapor Pressure• Vapor Pressure Lowering depends on mole fraction.

• Raoult’s Law:

Osmotic Pressure• Osmotic Pressure: Net movement of solvent (water) from

low concentration areas to high concentration areas across a semipermeable membrane.

Osmotic Pressure• Osmotic Pressure increases as MOLARITY increases

Boiling Point ElevationPure Solvent

Solution

Boiling Point ElevationPure Solvent

Solution

∆Tb=Tb(solution)-Tb(pure solvent) =iKbm

Freezing Point DepressionPure SolventSolution

Freezing Point DepressionPure SolventSolution

∆Tf=Tf(solution)-Tf(pure solvent) =iKfm

Example Problem: Freezing Point Depression

• Below is a table of van’t Hoff Factors (Kb and Kf) but water is missing!

• What is Kb if the freezing point of 500.0 g of water with 25.0 g of calcium chloride dissolved in it is 2.5 oC?

∆Tf=Tf(solution)-Tf(pure solvent) =iKfm

Example Problem: Boiling Point Elevation

• How many grams of salt (NaCl) would you have to add to 2.0 kg of pasta water to raise the boiling point 5oC?

• 1 tablespoon of NaCl is 25 g. How many tablespoons is that?

∆Tb=Tb(solution)-Tb(pure solvent) =iKbm

Example Problem: Osmotic Pressure

• What molarity of sodium chloride would a solution have to be to have an osmotic pressure equal to the osmotic pressure of blood which is 7.7 atm at 25 oC?

Example Problem: Vapor Pressure

• Sort the following by increasing vapor pressure of water:

• A) A solution 10.0 g of glucose (C6H12O6)

• B) A solution of 10.0 g of sucrose (C12H22O11)

• C) A solution of 10.0 g of methanol (CH3OH)