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Solutions. III. Colligative Properties. A. Definition. Colligative Property property that depends on the number (amount) of solute particles in the solution & not in the nature of the solute particles. Colligative Properties. Vapor Pressure Lowering Boiling Point Elevation

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a definition
A. Definition
  • Colligative Property
    • property that depends on the number (amount) of solute particles in the solution & not in the nature of the solute particles
colligative properties
Colligative Properties
  • Vapor Pressure Lowering
  • Boiling Point Elevation
  • Freezing Point Depression
  • Osmotic Pressure
vapor pressure lowering
Vapor Pressure Lowering
  • Vapor Pressure- pressure exerted by the vapor/gas above the liquid
  • VP of sol’n < VP of solvent

P = P1º-P1 = X2P1º


P1º = V.P. of solvent X2 = mole frxn of solute

P1 = V.P. of solution

boiling point elevation
Boiling Point Elevation
  • Boiling Point- the temperature at which the vapor pressure equals the external atmospheric pressure.
  • B.P. of sol’n > B.P. of solvent

Tb= Tb-Tbº = i Kbm

Tbº = B.P. of solvent Kb = molal B.P. constant

Tb = B.P. of solution = (0.52 °C/molal for H2O)

m = molality of sol’n i = van’t Hoff Factor

van t hoff factor i
van’t Hoff Factor, i

i = actual # of particles in sol’n after dissociation

# of formula units initially dissolved in sol’n

  • Nonelectrolytes (covalent)
    • remain intact when dissolved
    • 1 particle
  • Electrolytes (ionic)
    • dissociate into ions when dissolved
    • 2 or more particles

Boiling Point Elevation

Solute particles weaken IMF in the solvent.


- Fractional Distillation

freezing point depression
Freezing Point Depression
  • Freezing Point- the temperature at w/c a liquid phase is converted to the solid phase
  • F.P. of sol’n < F.P. of solvent

Tf= Tfº - Tf = i Kfm

Tfº = F.P. of solvent Kf = molal F.P. constant

Tf = F.P. of solution = (1.86 °C/molal for H2O)

m = molality of sol’n i = van’t Hoff Factor

    • salting icy roads
    • making ice cream
    • antifreeze
      • cars (-64°C to 136°C)
      • fish & insects
osmotic pressure
Osmotic Pressure
  • Osmosis- the selective passage of solvent molecules from a semi-permeable membrane from a dilute solution to more concentrated one.
  • Osmotic Pressure- pressure needed to prevent osmosis.

 = i MRT

 = osmotic pressure T = absolute temp

M = molarity of sol’n i = van’t Hoff Factor

R = 0.082058 L-atm/mol-K

Isotonic sol’n – sol’ns w/ same osmotic pressure / con’c
  • Hypotonic sol’n – the sol’n w/ lower con’c & osmotic pressure
  • Hypertonic- the sol’n w/ higher con’c & osmotic pressure

- Reverse Osmosis (Desalination of Water)

  • The effects of Colligative Properties on ELECTROLYTE soln’s are GREATER compared to NON_ELECTROLYTE soln’s.

1. What are the B.P. & F.P. of a 2.47 molal sol’n of naphthalene in benzene, (naphthalene does not ionize in benzene), given the ff data for benzene:

BP = 80.1 °C Kb = 2.53 °C/molal

FP = 5.5 °C Kf = 5.12 °C/molal


2. A sol’n containing 0.8330 g of a non-dissociating polymer of unknown structure in 170.0 mL of an organic solvent was found to have an osmotic pressure of 5.30 mmHg at 25°C. Determine the molar mass of the polymer.

3. The freezing point depression of a 0.100 M MgSO4(aq) sol’n is 0.225 °C. Calculate the van’t Hoff Factor of MgSO4(aq) at this con’c.


4. How many grams of Urea [(NH2)2CO] must be added to 450 g of H2O to give a sol’n w/ a V.P. of 2.50 mmHg less than that of pure H2O at 30°C. (V.P. of H2O at 30°C is 31.8 mmHg)

5. The average osmotic pressure of seawater is about 30.0 atm at 25°C. Calculate the molar con’c of an aqueous sol’n of Urea [(NH2)2CO], that is isotonic w/ seawater.