Lecture 36 12/5/05

Colligative Properties

Physical property

Based on concentration of solute molecules

Doesn’t depend on identity

Colligative Properties

Vapor pressure depression

Boiling point elevation

Freezing point depression

Osmotic pressure

Vapor Pressure depression

Raoult’s LawP = XsolventP°

ΔP = XsoluteP°

Fig. 15-15, p.742

Boiling Point elevation

ΔTb=Kbmi

m = molality = moles solute / kg solvent

What about boiling water for pasta?Kb (water) = 0.512 °C/m

Freezing Point Depression

ΔTf = Kfmi

Salting the sidewalksKf (water) = 1.86 °C/m

ethylene glycol

Coolant Pure Water50/50

C2H6O2/H2O70/30

C2H6O2/H2O

Freezing Point 0°C/32°F -37°C/-35°F -55°C/-67°F

Boiling Point 100°C/212°F 106°C/223°F 113°C/235°F

Osmotic pressure

Π = MRTi

Fig. 15-19, p.749

Reverse Osmosis