Chapter 14: Sec. 4 Colligative Properties of Solutions NOTES

Colligative Properties
o Colligative Properties:

4 Colligative Properties
 (1)
 (2)
 (3)
 (4)

(1) Vapor Pressure Lowering
o Vapor pressure: pressure exerted in a closed container by particles that have escaped the
liquid’s surface and entered the gaseous state.
o Adding a _____________________________ solute (one that isn’t likely to form into a gas) will
________________________ the vapor pressure.
 Nonvolatile solutes take up part of the surface area that would be formed into a gas if
the solute weren’t there
 The __________________________ the number of solute particles in a solvent, the
________________________ the vapor pressure – this is what makes it a colligative
property (depends on the number of particles)
 IMPORTANT: Ionic compounds will lower the vapor pressure more than molecular
compounds.
 The more ions formed the greater the vapor pressure lowering:
o Ex) NaCl = 2 ions; CaCl2 = 3 ions; AlCl3 = 4 ions

(2) Boiling Point Elevation
o Boiling Point Elevation:
o Boiling occurs when the vapor pressure of a liquid equals the atmospheric pressure
 Adding a nonvolatile solute lowers the vapor pressure and therefore raises the boiling
point
o Boiling Point Elevation Equation:




ΔTb =
Kb =
m=
Units =
o Value of the boiling point elevation is directly proportional to the solution’s solute molality
 The greater the number of solute particles in a solvent, the greater the boiling point
elevation – this is what makes it a colligative property (depends on the number of
particles)

(3) Freezing Point Depression
o Freezing Point Depression:
o Solute particles interfere with the attractive forces between solvent particles which prevents
the solvent to freeze
o Ex) Adding salt to ice or ice cream
o Freezing Point Depression Equation:
 ΔTf =
 Kf =
 m=
 Units =
o The greater the number of solute particles in the solvent, the greater the freezing point
depression – this is what makes it a colligative property (depends on the number of particles)

(4) Osmotic Pressure
o Osmosis:


Semipermeable membranes allow some particles, but not all, to cross
In osmosis only water travels across the membrane from an area of LOW concentration
of solute to an area of HIGH concentration of solute
o Osmotic pressure:

Depends on the number of solute particles in a given volume of solution – this is what
makes it a colligative property

BP and FP Nonelectrolyte Example:
o What are the boiling point and freezing point of a 0.40m solution of sucrose in water?

BP and FP Electrolyte Example:
o What are the boiling point and freezing point of a 0.16m aqueous solution of calcium chloride
(CaCl2)?