Colligative Properties

Colligative Properties• Doesn’t depend on identity, but number of particles• Boiling Point Elevation (BPE) – addition of solute raises Boiling Point• Freezing Point Depression (FPD) – addition of solute lowers Freezing Point• The reason for this is a bit complicated, just know that the addition of a solute

lowers the vapor pressure, thus changing the BP and FP• Antifreeze!

– What is the purpose of antifreeze?– Is the proportion of antifreeze to water important? YES!

Keeps water in radiator from freezing AND keeps it from

boiling when engine is running

FORMULAS for Colligative Properties

Molality = moles of solute ∆FP = mKf i kg of solvent ∆BP = mKb i

Each solvent is affected differently by the addition of solute particlesMolal freezing point constant for water, Kf = 1.86°C/mMolal boiling point constant for water, Kb = .512°C/m

The number of particles dissolved in the solution is what matters, not the identity of the particles

Molecular substances dissociate from each other, but remain as discrete molecules

Ionic substances break apart into ions, so each unit actually releases at least 2 particles (ions) into solutionC6H12O6(s) C6H12O6(aq) 1 particleNaCl(s) Na+

(aq) + Cl-(aq) 2 particles

CaCl2(s) Ca+2(aq) + 2Cl-

(aq) 3 particles

i

Other units of concentration

• % by Volume = volume of solute x 100% volume of solution• % by mass = mass of solute (g) x 100% mass of solution• Molality, m = moles of solute kg of solvent• Parts Per Million (ppm)