Question

Isolation and Recrystallization of Aspirin Experiment. If the melting-point range of my purified aspirin is 129 - 131 degrees Celsius, and pure aspirin melts at 135 degrees Celsius. Which of the following best describes the purity of the aspirin I isolated?

a. Very pure.

b. More pure than the impure sample but still contains impurities.

c. Very impure.

Explain

Answer 1

The melting point of pure compounds usually has a sharp melting point with a variation of 1-2 ^oC.
In this case, we have melting point about 4-6^oC variation compared to the pure compound.
A) very pure - Since the purified compound has a 4-6^oC variation to the pure compound, this compound is not pure.
Therefore, A is not correct.

B) More pure than the impure sample but still contains impurities.
The lowering melting point would be from 2-20^oC depending the impurities we have in the sample.
Usually, if the lowering melting point is > 5^oC, it is considered as an impure sample. Here, we are in the limiting range.
However, on average, the melting point is lowered by 5 ^oC. Therefore, this option is correct.

C) Very impure.
Since the lowering of melting point is an average ~5^oC, this compound is not considered as very impure.
Therefore, this option is not correct.

Hence, the correct answer is: B