1. Colligative Properties
Lecture 7.7
Colligative Properties

2. I. Colligative Properties
Properties that depend only on the #, and not the identity, of the solute particles.

3. II. Boiling-Point Elevation
Nonvolatile solutes raise BP of solvent.

4. II. Boiling-Point Elevation
Solutes lower VP of solvent.
Solutions must reach higher Ts to reach a VP of 1 atm.

5. II. Boiling-Point Elevation
ΔT = Kbmsolute
Kb is the molal boiling-point elevation constant.
Table of Kb values on pg. 505.

6. II. Boiling-Point Elevation
A sol’n is made by dissolving g glucose in g water. The sol’n has a BP of °C. What is the molar mass of glucose?
180.7 g/mol

7. III. Freezing-Point Depression
Solutes decrease the VP of the solution lower than that of ice.
ΔT = Kfmsolute
Kf is the molal freezing-point depression constant.

8. III. Freezing-Point Depression
Antifreeze

9. IV. Osmotic Pressure
Osmosis is the transfer of solvent particles through a semipermeable membrane.
Osmotic pressure is the minimum pressure that stops the osmosis.

10. IV. Osmotic Pressure

11. IV. Osmotic Pressure Π = MRT Π is the osmotic pressure in atm.
M is molarity
R is Latm/molK

12. IV. Osmotic Pressure
Ex. #2: The osmotic pressure of a solution containing mg of an unknown protein in mL of solution was torr at 25°C. Find the molar mass of the protein.

13. V. Electrolyte Solutions
Electrolytes dissociate.
Their effective concentration is dependent on the total number of ion particles.
For example 1 M NaCl would have 2 moles of ions in 1 L of water.

14. V. Electrolyte Solutions
The van’t Hoff Factor
i = moles of particles in sol’n
moles of solute dissolved

15. V. Electrolyte Solutions
Expected value for i: number of ions per formula unit.
Experimental value: slightly lower. See table on pg. 513.
This is due to spontaneous ion pairing.
More pronounced in concentrated solutions.

16. V. Electrolyte Solutions
Ex. #3: The observed osmotic pressure for a 0.10 M solution of Fe(NH4)2(SO4)2 at 25°C is atm. Compare the expected and experimental values for i.
Π = iMRT
4.4

17. PS 7.7
Read section
Ch. 11: #57-61 all, odd