Colligative Properties

Definition

• Properties that depend on the concentration of a solution no matter what the solute’s identity is

Boiling Point Elevation

• A solute lowers the vapor pressure, therefore a solution must be heated to a higher temperature in order to boil

• ΔT = KBm

• ΔT – amount of elevation

• KB – constant for a given solvent

• m – molality

Example

• A solution consisting of 18.00 g of glucose in 150.0 g of water elevates the boiling point to 100.34ºC. What is the molar mass of glucose?

Freezing Point Depression

• A solute lowers the vapor pressure of the solvent

• At lower vapor pressure, the rate of melting is higher than the rate of crystallization

• ΔT = KFm

Example

• What mass of ethylene glycol (C2H6O2), the main component of antifreeze, must be added to 10.0L of water to produce a solution in the car’s radiator that freezes at –23.3ºC?

Example

• A chemist is trying to identify a human hormone by determining it’s molar mass. A sample weighing .546 g was dissolved in 15.0 g of benzene, and the freezing point depression was determined to be .240ºC. What is it’s molar mass?