Colligative Properties
What are colligative
properties?
 Physical properties
 Based on # of solute particles, NOT identity!!!
 4 properties
1)
2)
3)
4)
Vapor Pressure Lowering
Freezing Point Depression
Boiling Point Elevation
Osmotic Pressure
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
Volatile vs. Nonvolatile
Liquids
1) Volatile Liquids
 WEAK intermolecular forces
 High vapor pressure
 Ex. Gasoline, alcohols, “Whoosh Bottle”
2) Nonvolatile Liquids
 STRONG intermolecular forces
 Low vapor pressure
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
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.
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)
Molality (m)
 Molality (m) = Moles solute
mass of solvent (kg)
Example 1
 A solution has 17.1g of sucrose (C12H22O11) dissolved
in 125g of water. Calculate the molal concentration of
the solution.
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
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)
Example 1:
 152g of sodium sulfate, Na2SO4, are dissolved in 875g
H2O. What is the freezing point of the solution?
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?
Homework
 Colligative Properties Worksheet #1-8
 Calculate molality and freezing point ONLY
3) Boiling Point Elevation
 Boiling point—
 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
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
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)
Example 1:
 If 90.0g of nonionizing glucluse (C6H12O6) are
dissolved in 255g of water, what is the resulting
solution’s boiling point?
Example 2:
 A solution contains 50.0g of sucrose (C12H22O11)
dissolved in 500.0g of water. What is the boiling-point
elevation?
Example 3:
 A solution has 450.0g of sucrose (C12H22O11) dissolved
in 250g of water. What is the solution’s boiling point?
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 !
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)
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
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.
Homework
 Colligative Properties Worksheet