Midterm IReview

Thursday, Feb. 08: Huggins 10, 7-8 pmConflicts: ELL 221, 6-7 pm

No class on Friday, Feb. 09!

13.26

The solubility of KNO3 is 155 g per 100 g of water at 75oC and 38.0 g at 25oC. What mass (in grams) of KNO3 will crystallize out of solution if exactly 100 g of its saturated solution at 75oC are cooled to 25oC?

Colligative properties

• Vapor-pressure lowering– Nonvolatile solute, ΔP = X2P1

o

– Does not apply to volatile solutes, PT = PA + PB (where PA = XAPAo)

• Boiling-point elevation– ΔTb = Kbm– Does not apply to volatile solutes

• Freezing-point depression– ΔTf = KFm– Applies to both volatile and nonvolatile solutes

• Osmotic pressure– Directly proportional to concentration of solution– As [solute] increases, so does osmotic pressure (pressure required to stop

osmosis, which is the net movement of solvent molecules from a pure solvent or dilute solution to a more concentrated solution)

Depend on amount of solute in solution, not on identity

Colligative properties

• You just dissolved a solute such as sugar in water. What happens to each of the following?

– Melting point (freezing point)• decreases

– Boiling point• increases

– Vapor pressure• decreases

– Osmotic pressure• increases

Depend on amount of solute in solution, not on identity

Using colligative properties to determine molar mass

Example 13.8

A 7.85-g sample of a compound is dissolved in 301 g of benzene. The freezing point of the solution is 1.05oC below that of pure benzene. What is the molar mass of this compound?

Solutions and solubility

• In most, but not all cases, solubility of a solid substance increases with temperature

• The solubility of gases usually decreases with temperature

• The solubility of gases usually increases with pressure– Henry’s law, c = kP

Effect of pressure on the solubility of gases

The solubility of N2 in blood at 37oC and at a partial pressure of 0.80 atm is 5.6 x 10-4 mol L-1. What is the value of Henry’s Law constant?

In M mmHg-1?

Henry’s law, c = kP

Concentration units

• % by mass of solute– Mass of solute/mass of solution, where mass of

solution is (mass of solute + mass of solvent)

• Molarity, M– Moles of solute/ volume of solution (L)

• Molality, m– Moles of solute/ mass of solvent (kg)

Concentration units• % by mass of solute

– Mass of solute/mass of solution, where mass of solution is (mass of solute + mass of solvent)

• Molarity, M– Moles of solute/ volume of solution (L)

• Molality, m– Moles of solute/ mass of solvent (kg)

Calculate the molality and molarity of a 30% by mass solution of NH3 in water. The density of the solution is 0.982 g/mL.

The rate constant• The rate constant k for a given reaction

depends on what parameters?

• The rate constant does NOT depend on the concentrations of reactants!

Rates of reaction

• For a first-order reaction, how long will it take for the concentration of reactant to fall to 1/8 its original value?

• Half-life of a 1st order reaction does not depend on the concentration.

Half-life t1/2 is the time it takes for the concentration of reactant to decrease to ½ its initial value

Rates of reaction

• A plot of ln [A] vs. time gives a straight line– 1st order

• A plot of 1/[A] vs. time gives a straight line– 2nd order

Determining reaction order graphically

Catalysts

• Rate constant– Increases

• Equilibrium constant– No change