Freezing Point Depression
Freezing Point DepressionLauric Acid
4/21/2011Tootoonchi FYThien Tran
Introduction
When a substance freezes or solidifies, the molecules come into
a certain arrangement. In some cases, the molecules become closely
packed to form a very dense solid [1]. If impurities are present,
more energy is used to solidify the substance, which lowers the
freezing point of the substance. The equation was used to determine
the freezing point depression constant (Kf) where T is the lowering
of the freezing point in C and m is molality. Molality is moles
solute/mass solvent. Molality is used instead of molarity because
volume (which is used in molarity) is dependent on temperature
whereas mass is not. Thus, a changing the temperature will not
change the molality in this case. The purpose of this experiment
was to determine the value of the freezing point depression
constant for Lauric acid. To find the freezing point depression of
Lauric acid, the melting point was determined twice for pure Lauric
acid and once each for 2 mixtures of Lauric acid and different
amounts of benzoic acid. Using the experimentally determined values
for the change in the melting point and the molality of the
samples, it was possible to find the freezing point depression
constant and take an average to determine the final value.
Procedure
First, a 400 mL beaker was filled with approximately 400mL of
water and placed a the hotplate until a temperature of about 60-70
degrees was reached while another 400 mL beaker with room
temperature water was placed to the side. Then, the LoggerPro
apparatus and the interface were connected to the computer and
started. After the water reached the desired temperature, a test
tube containing pure Lauric acid was placed into the heated water
bath until the entire sample had melted. Once the Lauric acid had
completely melted, the test tube was removed from the hot water
bath and the temperature probe was placed in the test tube and we
began to stir. After about 5 seconds, the test tube was then placed
into the other beaker and the data collection was started. The acid
was stirred until the temperature of the Lauric acid stopped
decreasing and leveled out. In order to continue the temperature
decrease, ice cubes were added to the beaker of water.To determine
the freezing point we highlighted to flat portion of the graph and
found the line of best fit for the temperature vs. time graph. The
y intercept of the linear fit line was the freezing point. Then,
the data was saved and stored and a new run was set up. The
procedure was performed again with the same sample of pure Lauric
acid. After each run, it was necessary to place the test tube with
the temperature probe back into the hot water bath to allow the
Lauric acid to melt so we could take out the temperature probe.
Then probe was then rinsed off and dried before using it with other
samples.After the first two tests of the pure Lauric acid, the same
test was run on two other samples. A test tube that contained 0.75
g of benzoic acid and 8.00 g of Lauric acid was placed into the hot
water bath to melt. After it melted, the freezing point was
determined the same way it was determined for the pure Lauric acid.
Then all the steps were repeated for a solution containing 1.50 g
of benzoic acid and 8.00 g of Lauric acid. Using the values for the
freezing point from the experiments, we were able to calculate the
T and the molality of each solution. Using this information and the
equation mentioned in the introduction (), we determined the
freezing point depression constant (Kf). The two values were then
averaged to find a final value for Kf.
Equations
EquationsVariables:
(1)
(2)
(3)
(4)
(5)
(6)
Data Collection
Mp of pure Lauric Acid 1 (C)43.7Mp of pure Lauric Acid 2
(C)43.45Average Mp for pure Lauric Acid (C)43.58Molecular Weight of
benzoic acid (g/mol)122.12Molality of 0.750g BA / 8.00g LA
(m)0.768T of 0.750g BA / 8.00g LA (C)1.09Kf determined from 0.750g
BA / 8.00g LA (C/m)1.42Molality of 1.50g BA / 8.00g LA (m)1.54T of
1.50g BA / 8.00g LA (C)7.07Kf determined from 1.50g BA / 8.00g LA
(C/m)6.01Average Kf (C/m)3.01
Calculations
The molality of the Lauric acid samples were calculated using
equation (2).
The average melting point temperature of the pure Lauric acid
was calculated using equation (3).
The change in temperature in each sample was determined using
equation (4).
The freezing point depression constant was determined using
equation (5) and rearranging the terms.
The average freezing point depression constant was calculated
using equation (6).
Analysis
How Error Can Be IntroducedError may have been introduced during
the cooling of the Lauric acid. While it was solidifying in the
beaker of water, the temperature probe was moving around and there
may have been an uneven temperature distribution since the acid was
cooling from the outside in. Therefore, touching the test tube with
the probe may have sent undesired readings to the collection of
data. Also, the test tubes were left open to the atmosphere while
warming up in the beaker on the hot plate. This may have allowed
the solution to evaporate some of its water content.
How This Error Will Change Our ResultIf the temperature probe
touched the outside of the beaker and picked up uneven readings for
temperature, the graph would have not leveled out nicely. There
would be slight bumps on the flat portion that would affect the
determination of the y-intercept, skewing the data. The flat
section of the graph would have been unreliable and the melting
points would have been off. Also, if some of the water had
evaporated from the test tube before the data collection began, the
molality may have been altered.
How to Prevent This ErrorTo prevent this error in the future,
test tube may be placed in a clamp attached to a ring stand, so
that it will be steadier and easier to stir. This way, one may only
need to worry about handling the probe, rather than keeping the
test tube submerged in the beaker at the same time. Also, the test
tubes may be heated while having the caps on, preventing any vapor
from escaping during the process.
References
[1] Lab manual
