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CHEM1002 Reactivity and Function in Chemistry: Post-Laboratory Questions

This document contains questions from a post-laboratory assignment on the purification of benzoic acid by recrystallization. The student answers three questions: 1) Their product had a slightly higher melting point than literature values, possibly due to benzylbenzoate or phthalic acid impurities; 2) Additional reasons for a low yield could be failing to thoroughly rinse equipment or use an icy cold solvent for rinsing; 3) Rinsing crystals with cold solvent removes surface impurities stuck or absorbed to the crystals.

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56 views1 page

CHEM1002 Reactivity and Function in Chemistry: Post-Laboratory Questions

This document contains questions from a post-laboratory assignment on the purification of benzoic acid by recrystallization. The student answers three questions: 1) Their product had a slightly higher melting point than literature values, possibly due to benzylbenzoate or phthalic acid impurities; 2) Additional reasons for a low yield could be failing to thoroughly rinse equipment or use an icy cold solvent for rinsing; 3) Rinsing crystals with cold solvent removes surface impurities stuck or absorbed to the crystals.

Uploaded by

moon star angel
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Name:

Student Number:

CHEM1002 Reactivity and Function in Chemistry


Post-laboratory questions

Post-laboratory questions are due at 11.59pm the following week on the day before your scheduled
lab using the Turnitin submission link on Blackboard, i.e. six days after your lab. No late submissions
will be accepted. Make sure you bring a copy to the lab to help you during the lab session.

Experiment 1: Purification of benzoic acid by recrystallization


Include your answers in the space after each question.

1. Consider the melting point range of your product. How does it compare to the literature value
for benzoic acid, and do you think you were able to recover a pure sample? If not, what do
you think would be the most likely impurity in your sample?
- The melting point range of the product formed is 129°C – 130°C. When it is being compared
with the literature value, it is found to be higher than the stated value for benzoic acid, which
is 122.4°C. The impurities that most likely to reside in the product formed are benzylbenzoate
and phthalic acid as these species can dissolve easily in a minimal amount of hot solvent.

2. In pre-lab question 5, you were asked to comment on why Fred obtained a very low yield after
recrystallisation. Considering the experiment you’ve just completed, and the % recovery you
achieved, comment on any additional reasons Fred’s yield was so low and ways he could
improve it next time.
- Fred could rinse his filtrate vessel (perhaps beaker or conical flask) used thoroughly during the
cooling process as some residual crystals could left behind,
- Fred could use icy cold solvent as a rinsing medium for the crystal.

3. It was important to rinse the crystals with pure cold solvent after they had been collected by
vacuum filtration. What type of impurities are being removed in this step?
- Impurities that are stick or absorbed onto the surface of the crystals, and they can be
removed by dissolving them in a pure cold solvent.

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