Question

MCAT Mini-Tutorial: Colligative Properties

Answer 1

\({\bf{Colligative~Properties:}}\) properties of a substance, dependent on particle quantity not type

The four colligative properties covered here are: vapor pressure, boiling point elevation, freezing point depression, osmotic pressure

\({\bf{Vapor~Pressure:}}\) pressure caused by the evaporation and subsequent pressure of evaporated molecules on the surface of a liquid
- dynamic equilibrium between condensation and evaporation on a liquid surface
- Raoult's Law: Pvap = (X_solvent)*P_pure
pure solvent * the mole fraction of solvent = vapor pressure
- vapor pressure decreases upon addition of a solute b/c solute molecules interfere with vaporization

\({\bf{Boiling~Point~Elevation:}}\) increase in boiling point caused by the addition of a solute
ΔTf = iK_f * m
where K_f is the freezing point depression constant
m is molality (moles of solute/kg of solvent)
i is van't hoff factor (# of particles upon dissolution, ex. NaCl has i = 2, glucose has i = 1)

\({\bf{Freezing~Point~Depression:}}\) decrease in freezing point caused by the addition of a solute
ΔTb = iK_b* m
K_b = boiling point elevation constant

\({\bf{Osmotic~Pressure:}}\) pressure that would prevent the spontaneous flow of solution due to a difference in concentration
osmotic pressure = Π = MRT
T = temp in Kelvin, R is ideal gas constant (be careful with units, most likely you will be working with liters so R = 0.0821L*atm/molK), M is molarity (not molality)

Answer 2

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT