Colligative Properties

Colligative properties are the physical changes that result from adding solute to a solvent. Colligative Properties depend on how many solute particles are present as well as the solvent amount, but they do NOT depend on the type of solute particles, although do depend on the type of solvent.

Solvent molecules are attracted together to a certain degree. The tighter the attraction (called intermolecular force) the less molecules that will escape liquid into gas phase = vapor pressure

If the Solvent is attracted to the solute particles more than to the other solvent molecules, then the vapor pressure will lower because the solvent molecules are held more tightly in the liquid phase.

Example: Automotive antifreeze consists of ethylene glycol (C₂H₆O₂) a non volatile nonelectrolyte. Calculate the new boiling point of the water in a radiator if 250. g of ethylene glycol are added to 750. g H₂O in the radiator.

• Find the molality of solution

• m = moles solute/ kg solvent = moles C₂H₆O₂ / kg H₂O

m = 250. g / 62.08 g/mol / 0.750 kg = 5.37 m

• ΔT_b = K_bmi = (0.51°C/m) (5.37 m)(1) = 2.74°C

• new bp = 100 + 2.74 = 102.74°C

Freezing Point

Example: The automotive antifreeze also alters the freezing point of the water in a car’s radiator. If 150. g of C₂H₆O₂ are dissolved in 850. g H₂O, what will the new freezing point of the solution be?

• Find the molality of solution

• m = moles solute/ kg solvent = moles C₂H₆O₂ / kg H₂O

m = 150. g / 62.08 g/mol / 0.850 kg = 2.84 m

• ΔT_f = K_fmi = (1.86°C/m) (2.84 m)(1) = 5.28°C

• new fp = 0 – 5.28 = -5.28°C

You may recall osmosis as being the movement of water through a semi-permeable membrane from Biology class. This is actually a colligative property.

Osmotic pressure, π, is the pressure required to stop osmosis which can be calculated using the following formula:

π = MRT = molarity x ideal gas law constant x Temp in K