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Determining correct pKas in organic chemistry

  1. Jan 25, 2007 #1
    1. The problem statement, all variables and given/known data
    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. Relevant 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.
     
  2. jcsd
  3. Jan 25, 2007 #2

    GCT

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    HCl has to have a pKa that is negative, tell me why.
     
  4. Jan 26, 2007 #3
    The higher the pKa, the more basic a substance is. HCl is a strong acid, therefore it should be negative... correct?
     
  5. Jan 26, 2007 #4

    GCT

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    Yep, tweak the pKa equation and see for yourself.
     
  6. Jan 26, 2007 #5
    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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