Colligative PropertiesColligative Properties

• Properties of solutions that depend on the concentration of solute particles. (Molarity or Molality!)

Colligative PropertiesColligative Properties

• Boiling Point Elevation & Freezing Point Depression

• Adding a solute to a liquid solvent can change the temperatures at which it boils or freezes.

(i)(K)(m)T

ΔT = The change in temperature

i = van’t Hoff factor (# of solute particles)

K = Constant (Pg. 522 boiling, 526 freezing)

m = Molality

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Colligative PropertiesColligative Properties

• Which has the higher van’t Hoff factor: NaCl or CaCl2?

• NaCl(s) Na+ (aq) + Cl- (aq)

• 2 particles

• CaCl2(s) Ca+2 (aq) + 2 Cl- (aq)

• 3 particles

Colligative PropertiesColligative Properties

• Example:

• What is the new freezing point of water when 117 g of NaCl is added to 500.0 g of water?

ΔT = (i)(K)(m)

1st Step – Find the molality

= 2.00 mol117 g NaCl 1 mol NaCl

58.5 g NaCl

= 4.00 mol/kg2.00 mol

.5000 kg

2nd Step – Find the van’t Hoff factor

NaCl(s) Na+ (aq) + Cl- (aq)

So….i = 2

ΔT = (2)(K)(4.00)

ΔT = (i)(K)(4.00 mol/kg)

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ΔT = (2)(K)(4.00)

Colligative PropertiesColligative Properties

• ΔT = (2)(1.86)(4.00)

• ΔT = 14.9 ºC

• But since H2O normally freezes at 0ºC, the freezing point is –14.9 ºC

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