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A chemist synthesised benzoic acid in the laboratory and purified it by recrystallisation - Leaving Cert Chemistry - Question (b) - 2010
Question (b)
A chemist synthesised benzoic acid in the laboratory and purified it by recrystallisation. The melting point of the product was measured before and after recrystalli... show full transcript
Worked Solution & Example Answer:A chemist synthesised benzoic acid in the laboratory and purified it by recrystallisation - Leaving Cert Chemistry - Question (b) - 2010
Step 1
Describe with the aid of a labelled diagram how the melting point of either the crude product or the recrystallised product could have been measured.
Answer
To measure the melting point of the benzoic acid, the sample is placed in a melting point tube and immersed in a heating block or a melting point apparatus. A thermometer is fixed in close proximity to the melting point tube to monitor the temperature. The melting point is determined by observing the temperature at which the substance fully transitions from solid to liquid.
The key components of this setup are:
- Melting Point Tube: A sealed tube where the sample is placed.
- Heating Block: Provides uniform heat to the tube.
- Thermometer: Attached to monitor the temperature closely during the process.
It's crucial to ensure that the temperature range is noted, as this indicates the specific melting point and any impurities present will broaden the melting range.
Step 2
Give two ways the chemist could have concluded from the melting point measurements that the recrystallised product was purer than the material first prepared.
Answer
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Higher Melting Point: The melting point of the recrystallised product will be higher than that of the crude material if purification has been successful because impurities typically lower the melting point.
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Narrower Melting Range: The recrystallised product will exhibit a sharper melting point range compared to the crude product, which indicates increased purity. A sharp range suggests a homogenous sample, while a broader range indicates the presence of impurities.
Step 3
Name the instrumental technique that could have been used to confirm the identity of the product based on its absorption of a unique set of low energy electromagnetic radiations.
Answer
The instrumental technique that could have been used is Infrared (IR) Spectroscopy. This technique identifies functional groups within the compound by measuring the absorbance of infrared radiation.
Step 4