David Goldfarb
Wednesday, February 03, 2010
CHEM-2290 Experimental Chemistry II
Lab 1: Crystallization in Single and Mixed Solvent with and without Activated Charcoal
Introduction: Solubility is based on the ability of one substance to dissolve another. The rule ‘like
dissolves like’ is applied to solubility, polar solvents will easily dissolve polar solutes and likewise for
non-polar substances. The method of crystallization and recrystallization is a way to purify substances.
As a solution is heated, more solute will go into solution as the solution becomes saturated. On cooling,
the compound’s solubility decreases leading to it crystallize (forming a mass of solid). These crystals are
purer than the original substance because from one seed crystal of pure substance, only pure crystal will
form. The impurities will remain in solution longer, or would have been removed prior to crystallization
(they would be less soluble).
There are many methods of crystallization, different solvents and solvent systems play a large
role in the crystallization process. In this lab the solubility of benzoic acid was analyzed in various
solvents, and then several crystallizations were carried out: impure benzoic acid in water, pure benzoic
acid in toluene-hexane, naphthalene in methanol-water. A decolorizing test was also performed.
Data and Results:
Solubility of Benzoic
Acid in Various
Solvents
At Room Temperature
In Hot Solution
Crystallization on
Cooling
Water
Insoluble
Slightly Soluble
Yes
Toluene
Slightly Soluble
Soluble
Somewhat
Hexane
Insoluble
Soluble
Yes
Mass of Impure Benzoic Acid used: 0.944g
Mass of Pure Benzoic Acid used: 0.984g
Mass of Naphthalene used: 2.058g
Mass of Benzoic Acid from crystallization (in water): 0.379g (40.15% recovery [.379/.944 *100%])
Mass of Benzoic Acid from crystallization (in toluene-hexane): 0.271g (27.54% recovery
[.279/.984*100%])
Mass of Naphthalene from crystallization: 0.3829g (18.61% recovery [.3829/2.058 *100%])
Discussion and Conclusion: It was obvious that benzoic acid was more soluble in some solvents than in
others, but through the crystallizing procedure it was dissolved and purified. Though not a large amount
of the product was recovered, it was quite pure. The samples dissolved and crystallized well during the
procedure, there was some that did not but it was probably mostly impurities. This was removed by
gravity filtration, so it makes up for some of the lost product. Based on the melting points of the final
products, the crystals were significantly more pure and well-formed.
The main mistake I made was just general rushing, due to lack of preplanning and inexperience, I
wasn’t able to allow the reaction as much time as it needed. I also didn’t pay as much attention to
recording data and observations as I should have; this led to some unclear ideas as to how well the
experiment went. These are both problems that can be improved upon later, and do not reflect on this
lab specifically. Despite these mistakes, the concepts addressed in the lab remained. The procedure of
recrystallization for purification was clearly demonstrated. The Benzoic Acid and Naphthalene were
successfully collected, and in appreciable amounts.
Experiment 1 Questions:
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Melting Point of Purified Benzoic Acid (from water): 118°C.
This is reasonably close to the established melting point of Benzoic Acid which is 122°C. This
would indicate that the sample was reasonably pure. It was very close to the measured melting
point of the crystalline product of the pure Benzoic Acid, this was 119°C.
The Percent Recovery of the Pure Benzoic Acid was only 27.54%. This is significantly lower than
the percent recovery of the impure Benzoic Acid. This could be because of the different solvent
system used for this recrystallization. Benzoic Acid was more soluble in these solvents at room
temperature. Therefore, as it cooled it did become less soluble, but not as insoluble as it did in
water. This probably hindered crystallization to an extent. It’s also possible that some of the
pure Benzoic acid stayed in solution during the filtration process, and did not precipitate or
crystallize.