Sections 11'5 and 11'6

1
Lecture 3

• Sections 11.5 and 11.6

2
First, the Questions

• What is the equation used to calculate the
  boiling point elevation for a solution of a
  nonvolatile nonelectrolyte?
• What is the equation used to calculate the
  freezing point depression for a solution of a
  nonvolatile nonelectrolyte?
• What happens to the boiling point of a solution
  of a nonvolatile solute, compared to the boiling
  point of the pure solvent?

3
More Questions

• What happens to the freezing point of a solution
  of a nonvolatile solute, compared to the freezing
  point of the pure solvent?
• Which species (solvent or solute) can pass
  through a semipermeable membrane used to measure
  osmotic pressure?
• What is able to pass through the membrane in
  dialysis?

4
One Last Question

• What would happen to red blood cells if pure
  water were given to a patient in an IV?
• (Have you heard of the sometimes fatal condition
  called water intoxication?)

5
Now, Your Questions

• ?

6
Section 11.5

• Effect of changing vapor pressures of solutions
  on boiling points and freezing points
• Remember, boiling point is temperature at which
  vapor pressure pressure of environment (p.
  466).
• If the solute is more volatile than solvent
• Vapor pressure of solution increases
• PT PA PB XAPA0 XBPB0
• Boiling point decreases
• The book does not address this situation.

7
Boiling Point Elevation

• If the solute is nonvolatile
• Vapor pressure of the solution decreases
• Psoln PA XAPA0
• Since XA is always less than 1 in a solution, the
  vapor pressure of the solution is less than the
  vapor pressure of the pure solvent.
• Boiling point increases
• ?Tb Kbmsolute Tb, solution Tb, solvent
• Equation valid in dilute solutions only,
  nonvolatile solutes only.

8
Freezing Point Depression

• Two important points
• When a solution freezes, crystals of pure solvent
  form, leaving solute in solution.
• At the freezing point (melting point), solid
  solvent has the same vapor pressure as the
  solvent in the solution (p. 465).
• Psolid Psolvent XsolventP0solvent
• Since Xsolvent is always less than 1, Psolvent is
  always less than P0solvent
• To be in equilibrium, Psolid must be lower than
  if pure solvent were freezing.
• The freezing point is lowered.
• Even if the solute is volatile!

9
Freezing Point Depression

• ?Tf Kfmsolute Tf, solvent Tf, solution
• Equation valid in dilute solutions only
• Compare equations carefully
• ?Tb Kbmsolute Tb, solution Tb, solvent
• Since values of Kb and Kf in book are both
  positive, you have to remember that in one case
  its an elevation and in the other case its a
  depression.
• For nonvolatile solutes, that is.

10
Section 11.6

• Solvent passes through semipermeable membrane
• From high solvent concentration to low solvent
  concentration
• Solvent flow stopped by applied pressure (Figure
  11.17) or by pressure of height difference of
  column (Figure 11.18)
• ? MRT
• R 0.08206 atmL/(molK)

11
Applications of Osmosis

• IV fluids must be isotonic
• Water intoxication
• Electrolyte replacement in sports drinks
• Water purification by reverse osmosis
• Forces water through the membrane
• Leaves solute behind
• Determining molar mass of solutes
• Sample Exercise 11.11
• Preparing isotonic IV fluids
• Sample Exercise 11.12