Colligative Properties

• Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles.

• Among colligative properties areVapor pressure lowering Boiling point elevationMelting point depressionOsmotic pressure

How many particles?

• All covalent molecules have 1 particle– All nonmetals– Sugar– Ethanol

• Ionic compounds you must count the # of ions.– Has a metal– NaCl=2 particles– BaCl2 =3 particles

Vapor Pressure

As solute molecules are added to a solution, the solvent become less able to evaporate(=decreased vapor pressure).

Boiling Point Elevation and Freezing Point Depression

Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.

Boiling Point ElevationThe change in boiling point is proportional to the molality of the particles in solution:

Tb = Kb m p

where Kb is a constant dependent upon the solvent.

Kb for solutions of water is 0.51

P is particlesTb is added to the normal boiling point of the solvent.

Freezing Point Depression• The change in freezing

point can be found similarly:

Tf = Kf m p

• Kf is a constant dependant upon the solvent.

• Kf for water is -1.86• Kf and Kb are found on

pg 438. Tf (a negative number) is added to the normal freezing point of the solvent.

Colligative Properties

• Consider three beakers:– 50.0 g of ice– 50.0 g of ice + 0.15 moles NaCl– 50.0 g of ice + 0.15 moles sugar (sucrose)

• Which will have the lowest freezing point?

• Which will have the lowest boiling point?

Colligative Properties - Osmosis

• Osmosis: the net movement of a solvent through a semipermeable membrane toward the solution with greater solute concentration.

• Movement of solvent from high solvent concentration to low solvent concentration

Osmosis plays an important role in living systems:

Membranes of red blood cells are semipermeable.