Question about AAMC Test 10 (bio, please help)

[Swarlos]

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This is in passage 2, Question 101:

1 propanol and 1-2 ethanediol can be seperated by which technique?

a. extraction with ethanol
b. extraction with H20
c. Recrystilzation
d. Fractional distillation

The answer is D. However, I was under the impression that Frac.Distillation can only be used if the boiling points of the 2 substanaces are <25 degrees apart. The BP for 1 propanol is 97 degrees and the BP for 1-2 ethanediol is 197 degrees..a difference of 100!

Can someone please help explain this to me?

I apologize that I can't post more info, its a passage based question. Once again that's in AAMC Prac Test 10, passage 2, question 101.

Thanks

 

[bananaz]

Fractional distillation separates mixtures by boiling point, whether the difference in BP is 25C or 150C. It is especially useful for compounds with similar BPs since those are difficult to separate via simple distillation, but it still works for substances that have significantly different BPs.

 

[cycloethane]

I now think that Recrystallization is only used when purifying solids. However, wouldn't it be possible take solids of those chemicals, heat them up, then cool them down and let the crystals form. The resulting crystals will be those of 1-Propanol.

 

[SN2ed]

Thread moved. These types of questions belong in the Study Q&A forum.

 

[mejorization]

Don't think seperating by melting point is a good idea. The melting points don't necessarily follow the trends of the boiling points. In this case, however, I think it does. Even so, it will be difficult to seperate because liquid can be trapped between crystals. Think of putting a bottle of soda in the freezer.

I now think that Recrystallization is only used when purifying solids. However, wouldn't it be possible take solids of those chemicals, heat them up, then cool them down and let the crystals form. The resulting crystals will be those of 1-Propanol.

 

[ahussani]

As someone stated, recrystalization is useful only for solids with melting points well above room temperature.

Neither extractions will work, because the technique requires that the compounds have different solubilities. Both compounds are highly soluable in water and ethanol.