[Sis290025]

1 For the following melting points, indicate what might be concluded regarding the purity of the sample:
a)120-122 °C   
b)147 °C
c)46-60°C
d)162.5-163.5  °C

I am using the idea that a pure organic compound melts in a range of 2 degrees or less.

a. Sample is slightly impure.
b. Sample is pure.
c. Impure sample
d. Slightly impure

2 Suppose that the observed melting point range of a solid was originally 150-160 °C, but this became 145-145.5 °C after recrystallization. How might you account for this observation?

There is a narrower range because recrystallization probably purified the compound?


Thanks.

[enahs]

Question 1)
All are “pure” except C.
If you look up common organic solid melting points of pure substances have a range for melting of 1-2 degrees.

Question 2)
You are correct. Except for simple filtering, recrystallization is the most common method for purifying substances.

[mike]

I think your answers to 1 are pretty much correct.

For question 2, your answer would be basically correct, a narrower melting pont range indicates a purer substance. The melting point range is also lower than the orginal so you have probably removed a higher melting point impurity. Alternatively a lower melting point could indicate the presence of solvent left over in the sample from insufficient drying after the recrystalisation.

I think you have the right idea, good work.

[wzfan]

you can detect it by TLC