Determining the Molecular
Mass by Freezing Point
Depression
By: Justin Green
Freezing Point Depression

Freezing point
depression is the
decrease of the
freezing
temperature of a
solvent after a
solute has been
added
www.chemprofessor.com
This shows the freezing point of
GEOMELT before and after adding
a solute. The solute lowers the
freezing point.
Freezing Point Depression


Ex. The freezing point of salt water is lower than that of pure
water
http://www.princeton.edu/~pccm/outreach/scsp/water_on_e
arth/salt/thermoms.gif
Molar Mass

The mass of one mole (6.022 x 1023
molecules) of any substance:
• Is given on the Periodic Table of
Elements
Formulas
Molar Mass can be found through the formulas following with
Freezing Point Depression:
M = wKf
w= mass fraction of solute
ΔT Kf = cyroscopic constant
ΔT = change in temperature
ΔTf = iKfm
I = van’t Hoff value
Kf = molal freezing point constant
m = concentration in molality
Formulas to use
ΔTf = I x kf x moles of solute
kg of solvent
ΔTf = I x kf x m
ΔTf = (g solute)
(MMsolute)
kg solvent
MMsolute = I x kf x g solute
kg of solvent x ΔTf
Purpose


To understand how freezing point
depression and molecular mass
correlate
To be able to use new knowledge of
freezing point depression in a real
world problem
Materials required





Medium sized test tubes
4.0g Naphthalene
52.0g paradichlorobenzene
Thermometer
Hot Water Bath (250 mL Beaker and 100 mL
water on Hot Plate)
Procedures:
Determining freezing point


1. Weigh out 4g of naphthalene
2. Weigh out 52g of
paradichlorobenzene
Procedures


3. Place test tube
containing
chemicals in hot
water bath
4. Place both
substances in the
hot water bath and
allow them to
completely melt
and stir well
http://www3.moe.edu.sg/edumall/tl/digital_resources/biology/images/hot_water_bath.jpg
Procedures

5. Remove from
the hot water bath
and allow it to cool
while stirring
gently to prevent
super cooling
http://cn1.kaboodle.com/hi/img/2/0/0/3e/c/AAAAAs-rj2IAAAAAAD7AHg.jpg
Procedures

6. Take the
temperature
(down to 0.1
degrees
Celsius) of
the solvent
every 60
seconds until
the solution is
solidified
http://wolfesscience.com/images/lab-thermometer.jpg
Freezing Point Graph
I drew this one myself 
Analysis Questions
1. What is the freezing point of the
final solution?
48.4 °C
2. What was the freezing point
depression of the solution? The
standard freezing point for
paradichlorobenzene is 53.0 degrees
Celsius
ΔTf= T°f – Tf = 53.0°C – 48.4°C =
4.6°C
Analysis Questions
3. What was the molality of the
naphthalene? The freezing point
depression constant for
paradichlorobenzene is 7.1°C x m-1
m = ΔTf = 4.6°C
= 0.65m
kf
7.1°C x m-1
Analysis Questions
4. What was the molecular mass of
naphthalene?
MMsolute= kf x g solute = 7.1°C x kg
kg solvent x ΔTf mol x .052 kg x 4.6°C
120 g mol-1
Analysis Questions
5. What was the percent error of the
results compared to the actual
molecular mass of 128 g x mol-1
120 – 128 x 100% = -6%
128
The results were 6% below expected
Conclusion

In conclusion, the molecular mass of
an unknown solute can be
determined through freezing point
depression
Freezing Point
Depression
By: Justin Green