Chapter 11 1

4. Colligative Properties: •Property of the solvent that depends on the number, not the identity, of the solute particles.•2 assumptions:

•no interactions between solvent and solute•dilute solutions

5 colligative properties:•solubility•vapor pressure lowering

solventpuresolutesolution PXP

Chapter 11 2

•Freezing point depression

fsolutensolfreeze KmT '.,

•Osmotic pressure,

TRnV TRM

where M is molarity

where Kf is a constant (see Table 11.5)

•Boiling point elevation

bsolutensolboil KmT '.,

where Kb is a constant (see Table 11.5)

Chapter 11 3

Vapor Pressure Lowering(Raoult’s Law, Ideal Solution Law)

solventpuresolutesolution PXP

Vapor more disordered than liquid

Pure liquid

Solution; more disordered than pure liquid

Lower drive to escape to disordered vapor because solution is more disordered than pure liquid

ENTROPY

Chapter 11 4

Example problem: What mass of urea, CON2H4, must be added to 450 g water to get a solution with a vapor pressure of 29.3 mmHg? The vapor pressure of pure water is 31.8 mmHg at this temperature.

•Write the appropriate equation (Raoult’s Law)

•write the known and unknown quantitiesknown: Psolution, Ppure solvent

unknown: X (what is X?)

•write out the definition of X

•write the known and unknown quantities

•solve for moles, then mass urea (how?)

Chapter 11 5

)()( BBAABAtotal XPXPPPP

where PA and PB are vapor pressures of A and B in the mixture, and PA

° and PB ° are vapor pressures of pure A

and pure B; XA and XB are mole fractions of A and B in the liquid

Chapter 11 6

Example: The vapor pressures of CCl4 and C6H12 are 0.284 and 0.246 bar, respectively. The vapor pressure of a mixture with X CCl4=0.5 is (0.530 bar / 0.265 bar / 0.018 bar).

answer:

Chapter 11 7

Question: Vapor pressure of the solution is (higher than / lower than / same as) the pure solvent; therefore the boiling point of the solution is (higher than / lower than / same as) the pure solvent.

Chapter 11 8

Freezing Point Depression: Question: The solution is (more / less) disordered than the pure solvent;therefore the freezing point of the solution is (higher than / lower than / same as) the pure solvent.

fsolutensolfreeze KmT '.,

l

S

Sol’n

S

solid is more orderedthan l or sol’n

sol’n less ordered than pure liquid:must go to lower temp. to freeze

liquid and solid are same substance

Chapter 11 9

time

apply pressure

sol’nmorediluteso moredisordered

Osmotic Pressure, / Osmosis

sol’n puresolvent

semi-permeablemembrane:lets solvent, not solutepass through

puresolvent

sol’n puresolvent

TRM

Entropy is the driving force!

Chapter 11 10

Molecular Cartoon of Osmosis

Question:A 1.0 M CaCl2 solution has an osmotic pressure (lower than / higher than / the same as) a 1.0 M glucose solution.

Chapter 11 11

read only.

Chapter 11 12

read only.