Colligative Properties Pre-Lab1. Units of Concentration

a. Molarity =

i. Volume dependentii. Temperature dependent; can’t use when T is changing

b. Molality =

2. Freezing Point Depression (Tf)a. Dissolved solutes lower the freezing point of liquids

i. Solute molecules block solvent from getting to the crystal to freezeii. The Equilibrium is shifted towards the liquid

Lmol

solution literssolute molesM

kgmol

solvent kilogramssolute molesm

b. Colligative Properties = depend only on the amount of solute, not the identity of the solutei. Freezing Point Depressionii. Boiling Point Elevationiii. Osmotic Pressure

c. Freezing Point Depression Equationi. kf = molal freezing point constant (different for each solvent)

ii. m = molality of the solution

)m)(k(ΔT ff

Temp

Time in ice bath

Pure Solvent (constant Tf)

Solution (Tf falls as concentration increases)Tf

3. Data Collection and Processinga. (Temp, time) data collected every 15 seconds (6 different runs)

i. Take your first point at room temp (before in bath) at time = 0 sii. Pure cyclohexane freezing pointiii. Cyclohexane + 1/5 of your solute sample (~0.16g)iv. Cyclohexane + 2/5 of you solute sample (~0.32g)v. Etc… for a total of 6 freezing point runs (0.48g, 0.64g, 0.80g)

a. Plot T vs. timeb. Use Excel to get 2 straight lines, which intersect at Tf

c. Tf = Tf(pure) – Tf(solution) [6 different plots; 5 different Tf]

4. Finding the Molecular Weight of the Unknown

bmxy

solvent kgsolute g

MWkΔT

solvent kgsoluteMW

solute g

)k(solvent kg

solute mol)k()m)(k(ΔT

ff

ffff

Force to 0

slopekMW

MWkslope ff

5. Units

6. Hints on doing the experimenta. First thing: make your ice bath as cold as possible by adding salt (-6 oC)

This will make each run go faster, saving you much time

b. Stirring during the freezing process is very important—demo

molg

kggC

kgmol

C

xyC/m20.1

slopekMW o

o

of