1. Colligative Properties

2. What are colligative properties?
Physical properties
Based on # of solute particles, NOT identity!!!
4 properties
Vapor Pressure Lowering
Freezing Point Depression
Boiling Point Elevation
Osmotic Pressure

3. 1) Vapor Pressure Lowering
Nonvolatile vs. volatile chemicals
Nonvolatile
Not likely to be a gas
Takes awhile for gas particles to evaporate
Volatile
Gas particles evaporate quickly
Vapor Pressure
Pressure exerted from gas molecules on a liquid
Characteristic of a liquid

5. Volatile vs. Nonvolatile Liquids
WEAK intermolecular forces
High vapor pressure
Ex. Gasoline, alcohols, “Whoosh Bottle”
Nonvolatile Liquids
STRONG intermolecular forces
Low vapor pressure

6. More Vapor Pressure…. INCREASED vapor pressure
Pure solvent
Particles can easily move between the liquid and gas phase to create an equal amount of particles in each phase (equilibrium)
DECREASED vapor pressure
Solvent + solute
Number of particles in the solution is increased
Less water/solvent can evaporate

8. 2) Freezing Point Depression
“difference between freezing point of a pure solvent and a solution containing a nonelectrolyte.”
Difference between
Freezing point of pure solvent AND
Freezing point of solvent + solute/substance
**Adding a solute/substance LOWERS the freezing point.

12. Molality (m) Another way to represent a solution’s concentration
Not as common as molarity
Used when determining colligative properties of solutions (boiling point elevation, freezing point depression)

13. Molality (m)
Molality (m) = Moles solute
mass of solvent (kg)

14. Example 1
A solution has 17.1g of sucrose (C12H22O11) dissolved in 125g of water. Calculate the molal concentration of the solution.
0.400mC12H22O11

15. Calculating Freezing-Point Depression
Δtf = iKfm
Δtf = change in freezing temperature (°C)
m = molality of solution
Kf = molal freezing point lowering constant (°C/m)
For water, 1.86 °C/m

16. Van Hoff factor (i)
Constant in the equation for freezing-point depression calculations
Most of the time, we will ignore this factor.
So equation become…. Δtf = -Kfm
Indicates how much dissociation has occurred in the solution
1 = compound does not dissocation
2 = compound dissociates into 2 ions (ex. LiCl)
3 = compound dissociates into 3 ions (ex. CaCl2)

17. Example 1:
152g of sodium sulfate, Na2SO4, are dissolved in 875g H2O. What is the freezing point of the solution?
-6.8°C

18. Example 2:
What is the freezing point depression (Δtf) of water in a solution of 17.1 sucrose (C12H22O11) in 200g of water? What is the actual freezing point of the solution?
Δtf = °C
Freezing point of solution = °C

19. Homework Colligative Properties Worksheet #1-8
Calculate molality and freezing point ONLY

20. 3) Boiling Point Elevation
Temperature where a liquid’s vapor pressure = atmosphere pressure
**Boiling point changes result from changes in vapor pressure.
“difference between boiling point of a pure solvent and a solution containing a nonelectrolyte”
Difference between
Boiling point of pure solvent AND
Boiling point of solvent + solute/substance

21. Calculating the Boiling Point Elevation
ΔTb = iKbm
ΔTb = change in boiling temperature (°C)
m = molality of solution
Kb = molal boiling point lowering constant (°C/m)
For water, 0.52 °C/m

22. Van Hoff factor (i)
Constant in the equation for freezing-point depression calculations
Most of the time, we will ignore this factor.
So equation become…. ΔTb = -Kbm
Indicates how much dissociation has occurred in the solution
1 = compound does not dissocation
2 = compound dissociates into 2 ions (ex. LiCl)
3 = compound dissociates into 3 ions (ex. CaCl2)

23. Example 1:
If 90.0g of nonionizing glucluse (C6H12O6) are dissolved in 255g of water, what is the resulting solution’s boiling point?
degrees C

24. Example 2:
A solution contains 50.0g of sucrose (C12H22O11) dissolved in 500.0g of water. What is the boiling-point elevation?
0.15 degrees C

25. Example 3:
A solution has 450.0g of sucrose (C12H22O11) dissolved in 250g of water. What is the solution’s boiling point?
102.7 degrees C

26. Boiling Point and Vapor Pressure
Substances with low vapor pressure
More energy needed for vapor pressure = atmospheric pressure SO
Boiling point INCREASES
** Solutes in a solution LOWER vapor pressure so boiling point INCREASES !

27. Osmosis Semipermeable membrane
Membrane selects what particles can go through and blocks other particles
Movement of water through a semipermeable membrane
Based on solute concentration (HighLow solute concentration)

30. 4) Osmotic Pressure
Pressure exerted by solute particles resisting osmosis
INCREASE osmotic pressure
High solute concentration
Water drawn in
DECREASE osmotic pressure
Low solute concentration
Water driven out

32. Electrolytes and Colligative Properties
Electrolytes GREATLY influence colligative properties.
Electrolytes---dissociate into ions in solution
Ions increase the NUMBER of solute particles in a solvent.

33. Homework
Colligative Properties Worksheet