Just retrieved my files way back in my undergraduate days. Might as well share it.the purpose of this experiment is to determine the apparent molecular weight of the solute from the lowering of freezing point and to determine the extent of dimerization and dissociation of acetic acid in benzene

1. Presented by:
•Janine V. Samelo
•BSChem 3
Groupmates:
•Nevah Rizza L.Sevilla
•Edessa Joy R. Sumagaysay
•Pauline Bianca R. Alfonso
•Jean Annerie M. Hernandez

2. Objectives:
1) To determine the apparent molecular
weight of the solute from the lowering of
freezing point.
2) To determine the extent of
dimerization and dissociation of acetic
acid in benzene

3. Introduction:
The properties of a solution differ from those
of a pure solvent due to interactions that take
place between the solute and solvent molecules.
The properties that exhibit such changes are
called the colligative properties and include vapor
pressure lowering, boiling point elevation,
freezing point depression and osmotic pressure
change. These properties are dependent only
upon the number of particles (ions or molecules)
which are dissolved in the solvent and not on the
identity of the particles.
Freezing-point depression, one of the colligative
properties of solution, is used in this experiment
to determine the molecular weight of an
unknown compound A. The temperature at which
a solution freezes will be lower than the
temperature at which the pure solvent freezes.

4. The difference between the freezing point of a
solvent (ΔTf) and the freezing point of the
solution (Tf) is referred to as the freezing point
depression (ΔT). A solution’s freezing point is
related to the molality (m) of the solution. The
equation that describes the relationship between
freezing point depression and molality is:
ΔTf = (Kf)(m)
ΔT represents the freezing point depression:
ΔT = Tf- Ti
Tf is the freezing point of the pure solvent.
Tf is the freezing point of the solution.
Kf is the molal freezing point depression
constant.
The value of Kf is characteristic for a given
solvent. The units of Kf are given in oC/m.
m is the molality of the solution.
m = moles of solute/ kg of solvent.

5. Insert the thermometer and stirring rod
inside the tube and place it in the ice bath.
Pipet out about 25.00 mL benzene into the
freezing point test tube. Record the its
initial temperature.
Fill the small Dewar flask with ice-water
mixture
Pre-chill the benzene in an ice bath.

6. Add four 0.05 mL glacial acetic acid
successively repeating steps 4 to 7 with
each addition.
Add 0.10 mL glacial acetic acid and repeat
the previous steps of taking temperatures.
Get the tube from the icebath and warm it
with hand until the crystals formed by
supercooling have melted.
Continue taking temperature readings at 30
sec-intervals for another 5 mins or when
the temperature becomes constant.
Stir steadily up and down and take
temperature readings at 30-second interval
until the temperature remains constant

8. Data and Calculations:
TEMPERATURE READINGS
BENZENE
After addition of glacial acetic acid
+ 0.05
mL
+ 0.10
mL
+ 0.15
mL
+ 0.20
mL
Initial temp:
24.4oC
20.2 oC 23.1 oC 20.1 oC 21.5 oC
30 s interval:
12.9 oC
14.3 13.6 13.9 14.6
8.4 8.2 7.8 7.7 8.3
6.4 6.4 6.1 5.5 6.3
5.5 5.2 5.6 5.0 5.5
5.0 4.5 4.1 4.5 4.3
4.4 4.0 3.8 4.0 4.1
4.1 3.5 3.5 3.2 3.3
4.1 3.6 3.5 2.1 1.9
4.1 3.6 3.2 2.1 1.5

9. Table 2: Calculated values for Van’t Hoff factor,
apparent molecular weight, extent dissociation and
polymerization after addition of concentrated glacial
acetic acid.
+ 0.05 mL
CH3COOH
+ 0.10 mL
CH3COOH
+ 0.15 mL
CH3COOH
+ 0.20 mL
CH3COOH
Δ Tf 0.5 0.9 2.0 2.6
Mass of acetic
acid
0.05245 g 0.10490 g 0.15735 g 0.20980 g
Molality
0.039893
m
0.079787 m
0.119681
m
0.15974 m
Van’t Hoff
factor ( i )
2.448 2.203 3.264 3.182
Molecular
weight
60.00189
g/mol
59.9964
g/mol
60.00189
g/mol
58.56455
g/mol
α
polymerization
-2.896 -2.406 -4.528 -4.364
α dissociation 1.448 1.203 2.264 2.182

10. Density of benzene: 0.8765 g/mL
Volume of benzene: 25 mL
Density of acetic acid: 1.049 g/mL
Kf of benzene: 5.12 0C/m
Mass of benzene: (vol of benzene x density of
benzene)
= = 21.9125 g
Mass of acetic acid: (volume of acetic acid x
density of acetic acid)
For 0.05 mL= = 0.05245 g
For 0.10 mL= = 0.10490 g

11. For 0.15 mL= = 0.15735 g
For 0.20 mL= = 0.20980 g
Molality of solution=
For 0.05 mL = = 0.039893 m
For 0.10 mL = = 0.079787 m
For 0.15 mL = = 0.119681 m
For 0.20 mL = = 0.159574 m

12. Van’t Hoff factor:
For 0.05 mL = = 2.448
For 0.10 mL = = 2.203
For 0.15 mL = = 3.264
For 0.20 mL = = 3.182
Apparent molecular weight:
For 0.05 mL =
= 60.00189 g/mol

13. For 0.10 mL = = 59.99644 g/mol
For 0.15 mL = = 60.00189 g/mol
For 0.20 mL = = 58.56455 g/mol
Extent polymerization: α =
For 0.05 mL = = -2.896
For 0.10 mL = = -2.406
F0r 0.15 mL = = -4.528
For 0.20 mL = = -4.364

14. Extent dissociation: α =
For 0.05 mL = = 1.448
For 0.10 mL = = 1.203
F0r 0.15 mL = = 2.264
For 0.20 mL = = 2.182

15. The freezing point of a liquid is depressed when it
contains a dissolved solid. The freezing point
depression, or the difference between the freezing
points of the pure solvent and solution, depends
upon the number of particles in solution. The size
of freezing point depression depends on two
things: a) the size of Kf for a given solvent, which
is well known, and b) the molal concentration of
the solution which depends on the number of
moles of solute and kg of solvent. The greater the
concentration of the solution, the greater will be
the freezing point depression.
In the experiment, as time passes and more
acetic acid is added in pure benzene, the freezing
point decreases.

16. 0
5
10
15
20
25
30
0 50 100 150 200 250 300
Temperature(C)
TIME (s)
Freezing point depression
pure benzene
1st addition
2nd addition
3rd addition
4th addition

17. Electrolytes have larger effects on boiling
point elevation and freezing point depression. For
a given concentration, a solute that dissociates
will also bring about a greater freezing point
depression. Acetic acid, a weak electrolyte, can
dissociate or dimerize depending on the solvent.
In aqueous solution, acetic acid dissociates into
H+ and CH3COO-, thus, giving twice as many
particles in solution.
Van't Hoff factor ( i ) is used to introduce the
effect of ion pairing or association of ions that
prevents the effect of being exactly equal to the
number of dissociated ions. It is the measure of
the extent of ionization or dissociation of the
electrolytes in the solution.

18. It has an ideal value of 3 for 2:1 electrolyte
like K2SO4 and CaCl2 and a value of 2 for 1:1
electrolytes like NaCl, KI, and also for acetic acid.
In organic medium, the acetic acid tend to
form dimers [(CH3COOH)2]. Dimerization occurs
when two similar molecules join together by
addition or condensation to form a larger
molecule. The values of i calculated was then
used to estimate the extent of dimerization of
acetic acid in benzene.

19. From the lowering of freezing point observed in
the experiment, the computed values for the
molecular weight of acetic acid are: 60.00189,
59.99644, 60.00189 and 58.56455 having an
average of 59.64118. With this, the percent
relative error from 60 g/mol molecular weight of
acetic acid is -0.598%.
The values for the degree of polymerization
for 0.05 mL, 0.10 mL, 0.15 mL and 0.2 mL are: -
2.896, -2.406, -4.528, and -4.364 respectively.
The computed values for the degree of
dissociation are: 1.448, 1.203, 2.264 and 2.182.

20. http://en.wikipedia.org/wiki/Freezing-po…
http://books.google.com.ph/books
http://wwwchem.csustan.edu
http://chemmovies.unl.edu
eee.uci.edu/programs/gchem/02MANfrzpt
dep.pdf
MBAgustin- Chem 110 supplementary
notes