Organic Chemistry
Posted at 07:14 PM on August 23, 2009.
Paranoid
Abstract
This
experiment was performed to determine the sublimation and melting point of benzoic
acid. The sublimation set-up was used to purify the benzoic acid and collect
its sublimate, which was required in determining its melting point and compare
it with a standard. Only .1g of the sublimate was obtained, thus making its
percentage recovery 2%. This just shows that the original 5g of impure benzoic
acid will only yield a small amount of sublimate. As for its melting point, the
sublimate had a 10°C range in temperature as compared to the benzoic acid with an
8°C range. Hence, it can be said that the sublimate is impure because there is
a wide melting point range.
Introduction
Sublimation
is a technique used by chemists to purify solid mixtures as long as the
impurities are non-volatile or have a lower vapor pressure compared to a pure
compound. Melting point on the other hand is the change in state of the
compound from solid to liquid. In this experiment, impure benzoic acid
undergoes sublimation to be purified, and the sublimate obtained is used for
the melting point determination.
The
objectives of this experiment are: to be able to purify the benzoic acid by sublimation,
to determine the melting point of the product so as to compare it with a
standard, and to calculate the percentage recovery of the sublimate.
Experimental
5
grams of impure benzoic acid was placed in an evaporating dish covered with a
perforated bond paper and a pre-weighed watch glass. A moist tissue was also
placed on top of the set-up to cool the center. The contents were then heated
for 10-15 minutes using a hot plate and were left to cool after heating. The
group then collected the sublimate and placed it on the pre-weighted watch
glass and was weighed again for determining the percentage recovery.
For
melting point determination, the sublimate was grinded into fine powder and was
placed in a capillary tube. The latter and the standard were then attached to a
thermometer using a rubber band and were immersed in an oil bath for heating.
The group recorded the temperature in which the sublimate and the benzoic acid started
to melt and when it has completely melted. Lastly, after both data were
obtained, both melting points were compared.
Results and Discussion
After
performing sublimation, only .1g of the solid
benzoic acid volatilized and was condensed as a
purified compound (sublimate) while the non-volatile residue impurities were left behind. Because of this, the group was able to obtain a percentage
recovery of only 2%.
As for the melting point of benzoic acid, the
temperature at which it started to melt was 126°C and it completely melted at a temperature of
134°C. On the other hand, the temperature at which the sublimate started to
melt was 122°C and it completely melted at 132°C. The range of the melting
point of benzoic acid is 8°C as compared to the sublimate’s 10°C.
Hence, the sublimate is considered impure because it has a bigger range as
compared to the benzoic acid.
The range is a basis for
determining if a substance is either pure or impure because if the range is
broad, more impurity is present in the substance. In contrast to if the range
is small, it can be said that the substance is pure. Furthermore, the melting
point of a pure substance is always higher than the melting point of an impure
substance.
References
Xavier
http://www.xula.edu/chemistry/department/organic/Notes/06MP.pdf
Melting
point. (2009, August 7). In Wikipedia,
the free encyclopedia. Retrieved August 14, 2009, from http://en.wikipedia.org/wiki/Melting_point
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