Colligative Properties Pre-Lab
1.
Units of Concentration
moles solute mol

a. Molarity = M 
liters solution
L
i.
ii.
b.
2.
Volume dependent
Temperature dependent; can’t use when T is changing
Molality = m 
molessolute
mol

kilogramssolvent kg
Freezing Point Depression (DTf)
a. Dissolved solutes lower the freezing point of liquids
i. Solute molecules block solvent from getting to the crystal to freeze
ii. The Equilibrium is shifted towards the liquid
b. Colligative Properties = depend only on the amount of solute, not the
identity of the solute
i. Freezing Point Depression
ii. Boiling Point Elevation
iii. Osmotic Pressure
c.
ΔTf  (k f )(m)
Freezing Point Depression Equation
i. kf = molal freezing point constant (different for each solvent)
ii. m = molality of the solution
Pure Solvent (constant Tf)
Temp
DTf
Solution (Tf falls as concentration increases)
Time in ice bath
3. Data Collection and Processing
a. (Temp, time) data collected every 15 seconds (6 different runs)
i. Take your first point at room temp (before in bath) at time = 0 s
ii. Pure cyclohexane freezing point
iii. 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. time
b. Use Excel to get 2 straight lines, which intersect at Tf
c. DTf = Tf(pure) – Tf(solution) [6 different plots; 5 different DTf]
Temp vs Time
45
Temperature (degrees C)
40
35
30
25
20
15
10
5
0
0
2
4
6
Time (min)
8
10
12
4.
Finding the Molecular Weight of the Unknown
  g solute  


 molsolute 
MW solute  
  (k f ) 
ΔTf  (k f )(m)  (k f )
 kg solvent 
 kg solvent




 k  g solute 

 ΔTf   f 
 MW  kg solvent
 y  m x b
Freezing Point Depression Chart
0.2
0.18
0.16
0.14
Delta T (degrees C)
y = 0.003x
0.12
0.1
0.08
slope 
0.06
kf
k
 MW  f
MW
slope
0.04
0.02
0
Force to 0
0
10
20
30
40
grams solute/kg solvent
50
60
5.
Units
6.
Hints on doing the experiment
a. 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
o
C
 mol


o
kf
20.1 C/m  kg 
g
MW 

 o

C
slope
mol
 Dy 


 g 
 Dx 
 
 kg 
b.
Stirring during the freezing process is very important—demo