TBR Organic Lab Techniques Passage I Question 2

[dennis-brodmann]

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I have a question from TBR Organic Lab Techniques Passage I ("Distillation and Separation") Question 2.

In the passage, they list three different compositions for a mixture that a student is trying to separate by fractional distillation.

1) Equal parts by volume of Z-1,2-dichloroethene (b.p. 60 deg C) and E-1,2-dichloroethene (b.p. 48 deg C).

2) Equal parts by volume of 3-pentanone (b.p. 102 deg C) and 2-hexanone (b.p. 128 deg C)

3) Equal parts by mass of diethyl ether (b.p. 34 deg C) and di-n-propyl ether (b.p. 90 deg C).

Question 2 says
For the experiment, the thermometer read 49.2 deg C as the first aliquot was collected. The compound being collected is most likely which of the following?

A. E-1,2-dichloroethene
B. Z-1,2-dichloroethene
C. Diethyl ether
D. 3-pentanone

I chose C because if the mixture included the diethyl ether, by 49.2 deg C, all of that liquid should have condensed since it is the most volatile.

The explanations say it was A though, since the b.p. is closest to E-1,2-dichloroethene.

What's wrong with my reasoning?

 

[member232]

Fractional distillation separates components based on boiling points-- thus, for diethyl ether, which has a boiling point at 34 , you would only need to heat the solution to 34 and a little bit higher to make all the diethyl ehter evaporate... the diethyl collected would then be allowed to cool as it is now separated. It doesn't really make sense to keep heating the solution of already separated diethyl ether to 49. On the other hand, heating the solution to 49 would be most practical to separate a solution with bp of 48, because that's the amount of heat you need to make it evaporate. In this case, the question is looking to test you on what is most practical. Yes, diethyl ether would evaporate at 49, but it makes more sense that you'd be separating E-1-2 dichloroethane at this temp.

 

[Jepstein30]

I have a question from TBR Organic Lab Techniques Passage I ("Distillation and Separation") Question 2.

In the passage, they list three different compositions for a mixture that a student is trying to separate by fractional distillation.

1) Equal parts by volume of Z-1,2-dichloroethene (b.p. 60 deg C) and E-1,2-dichloroethene (b.p. 48 deg C).

2) Equal parts by volume of 3-pentanone (b.p. 102 deg C) and 2-hexanone (b.p. 128 deg C)

3) Equal parts by mass of diethyl ether (b.p. 34 deg C) and di-n-propyl ether (b.p. 90 deg C).

Question 2 says
For the experiment, the thermometer read 49.2 deg C as the first aliquot was collected. The compound being collected is most likely which of the following?

A. E-1,2-dichloroethene
B. Z-1,2-dichloroethene
C. Diethyl ether
D. 3-pentanone

I chose C because if the mixture included the diethyl ether, by 49.2 deg C, all of that liquid should have condensed since it is the most volatile.

The explanations say it was A though, since the b.p. is closest to E-1,2-dichloroethene.

What's wrong with my reasoning?

"as the first aliquot was collected"

If it was diethyl ether, with a BP of 39 deg C, it would have already begun melting prior to 49.2 deg C.

 

[dennis-brodmann]

Oh ok. I guess I over thought my answer. Thanks guys!