Chemistry question..

[sweetpea2]

1. A .6M solution is made by dissolving compound X in water. After 10 secs the concentration of X is .4M. All of the following can account for this EXCEPT:

A Precipitation
B Neutralization
C Evaporation
D Decomposition
E Disproportionation

The answer is A, but i cant seem to reason it out..maybe b/c with ppt molarity??

2. And heres just an orgo question: Im under the impression that allylic carbocation is most stable followed by a tert. carbocation. Also, would three phenyl groups on a tert carbocation be more stable than 3 methyl groups..am i correct? Can someone please go through the whole stability thing b/w say phenyl vs methyl and allylic carbocation. Thanks in advance!

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[TXDDS2014]

Are you sure it is A, b/c if it precipitated out of solution it would decrease the molarity. I was thinking that evaporation would be the answer b/c if water evaporates it would cause the Molarity to increase.

For stability the first thing you look at is Benzyllic>Allylic>Normal>Vinylic. (acronym = BANV). So a primary benzyllic carbocation is more stable than a secondary, or whatever, allylic carbocation. If both carbocations are the same type then tertiary>secondary>primary. Also a carbocation with three phenyl groups is much more stable than one with methyls because the benzenes allow the positive charge to resonate.

 

[sweetpea2]

Yea the answer key says A, but this is the OAT exam that i found like 3 other mistakes. So i can never be sure..what you say actually makes sense..i dont know, its pretty sad that all these study guides/Actual exams have mistakes

 

[TexasDent]

Are you sure it is A, b/c if it precipitated out of solution it would decrease the molarity. I was thinking that evaporation would be the answer b/c if water evaporates it would cause the Molarity to increase.

This is exactly what I thought when I read the question. I am pretty sure that the only choice that would not decrease the Molarity is evaporation which would increase the molarity. So I agree, I think that the answer should be C (evaporation).