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Determining correct pKas in organic chemistry

  • Thread starter future_vet
  • Start date
169
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1. Homework Statement
This is a problem taken from an old exam:
C. Which set of pKa’s is correct?
1. CH3COCH3 -7.0 2. CH3COCH3 -19.0
CH3COOH 4.8 CH3COOH -4.8
HCl 19.0 HCl 7.0

3. CH3COCH3 4.8 4. CH3COCH3 19.0
CH3COOH 7.0 CH3COOH 4.8
HCl 16.0 HCl -7.0


2. Homework Equations
None that I know of.


3. The Attempt at a Solution
I know from looking at the pKa values we were given in the lecture than the right answer is #4. Nevertheless, I was wondering if there was a way to get to this answer without learning the pKas by heart (a reasoning that would make the question logical).

Thank you,

Joanna.
 

Answers and Replies

GCT
Science Advisor
Homework Helper
1,727
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HCl has to have a pKa that is negative, tell me why.
 
169
0
The higher the pKa, the more basic a substance is. HCl is a strong acid, therefore it should be negative... correct?
 
GCT
Science Advisor
Homework Helper
1,727
0
Yep, tweak the pKa equation and see for yourself.
 
44
0
A general rule that I learned in Ochem...know water (about 16)...strong bases will have a high pKa and a strong acids will have a low one. Weak acids will be close to water.
 

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