Chem-To-Go Lesson 37Unit 9

COLLIGATIVE PROPERTIES

COLLIGATIVE PROPERTIES

DEFINITION: a physical property of a solution that differs from the physical property of the solvent due to the presence of the solute particles; the number of solute particles determines the severity of the property’s change

BOILING POINT ELEVATION

The boiling point of a solution will be higher than the normal boiling point of the solvent.• EXAMPLE: Salt water boils

at a higher temperature than pure water.

FREEZING POINT DEPRESSION

The freezing point of a solution will be lower than the normal freezing point of the solvent.• EXAMPLE: Salt water

freezes at a lower temperature than pure water.

IONIC SOLUTESIonic solutes

dissociate as they dissolve creating MORE solute particles.

COVALENT SOLUTES

Covalent solutes do not dissociate as they dissolve.

NUMBER OF SOLUTE PARTICLES

∆Tf = Kf m i∆Tf = change in

freezing pt.K f = freezing point

constant for the solvent (will be given)

m = molalityi = number of ions

present in solute (USE ONLY WITH IONIC SOLUTES)

FREEZING POINT DEPRESSION

Kf of water = 1.86°C/m

What is the new freezing point when 15.0g NaCl is dissolved into 200 mL of

water?

∆Tb = Kb m i∆Tb = change in

boiling pt.Kb = boiling point

constant for the solvent (will be given)

m = molalityi = number of ions

present in solute (USE ONLY WITH IONIC SOLUTES)

BOILING POINT ELEVATION

Kb of water = 0.52°C/m

What is the new boiling point when 15.0g NaCl is dissolved

into 200 mL of water?

CIVIL ENGINEERING APPLICATION: How many grams of NaCl would need to be added to the water to change the freezing temperature of 200 mL to -8°C?

MORE ADVANCED EXAMPLE