Dr Collins Chemistry Question

[pharmboy27]

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Hey guys so I am studying for the July 9th PCAT (Wish me luck!) and I came across two questions that seem just wrong to me. They are on Chemistry Test #17

These are the questions

8. Consider the three isomeric alkanes n-hexane, 2,3-dimethylbutane, and 2-methylpentane. Which of the following correctly lists these compounds in order of increasing melting point
A. 2,3-dimethylbutane < 2-methylpentane < n-hexane
B. 2-methylpentane < n-hexane < 2,3-dimethylbutane
C. 2-methylpentane < 2,3-dimethylbutane < n-hexane
D. n-hexane < 2-methylpentane < 2,3-dimethylbutane
E. n-hexane < 2,3-dimethylbutane < 2-methylpentane

I thought the answer was E but the answer key say D with the explanation "branching = greater MP"

19. Of the following molecules, which would have the highest boiling point?
A. C(CH3)4
B. CH3CH2CH2CH2CH3
C. 2-methylbutane
D. n-pentane

So for this one B and D are the same and that's what I thought the answer was because I thought branching decreases boiling point, but the answer key say A with the explanation "most branching = greatest surface area"

Aren't these wrong?

 

[pharmboy27]

bump

 

[Apothecary2010]

my answer for question 8 about comparing melting points A is the correct answer.... you have to put it in increasing order so first less mp and medium and high mp compounds..
23, dimethyl compound is more branched than 2 methyl compound and the third is the straight chain, so more branching less melting and boiling points.


Here is the explanation from kaplan, "branched molecules have slightly lower boiling and melting points than their straight chain isomers. greter branching reduces the surface area of a molecule, decreasing the weak intermolecular attractive forces. The molecules are held together less tightly, thus lowering the boiling point. In addition, branched molecules are more difficult to pack into a tight tree dimensionla structure which is reflected in the lower melting points of branched alkanes."

The other question you are right B and D are the right answers......

 

[MyRealNameIs]

I think the above poster is wrong.

Less branching = higher boiling and lower melting point due to stronger bonds.

More branching = lower boiling point and higher melting point.

This is because its boiling point elevation and freezing point depression. Stronger molecular bonds result in higher boiling point and lower melting point.

I noticed Dr. Collins have wrong answers from time to time. I always just looked up boiling points of the molecules to confirm my suspicions online.