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Organic pKa

  1. Sep 27, 2009 #1
    1. The problem statement, all variables and given/known data
    Acid HA has a pKa=20; acid HB has a pKa=10
    a. which is the stronger acid?
    b. will an acid-base reaction with an equilibrium lying to the right take place if Na+ A- is added to HB? Explain your answer.

    2. Relevant equations

    3. The attempt at a solution
    I know that the lower pKa is a stronger acid, which makes answer a is HB
    I'm not really sure what b is asking. I want to say yes because HB is a strong acid, so adding Na+ A- (weak base) will cause an acid base reaction. I'm like clueless with b. Can someone help and tell me where to start ffor b? thank you in advance..
  2. jcsd
  3. Sep 27, 2009 #2


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    Staff: Mentor

    What it really asks if it this reaction will happen:

    HB + A- = B- + HA

    and to find the answer you have to know wich conjugate base - A- or B- is stronger.

  4. Sep 27, 2009 #3
    HB + A- = B- + HA

    strong acid + A- = B- + weak acid

    Since HB is a stronger acid, isn't A- a stronger base? stronger acid=weaker conj. base?
  5. Sep 27, 2009 #4
    so answer is yes
  6. Sep 27, 2009 #5


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    Staff: Mentor

    Yes it is yes :wink:

  7. Sep 27, 2009 #6
    thank you so to explain my answer :
    HB + Na+ A- = B- + HA

    Since HB is a stronger acid, A- is a stronger base, stronger acid=weaker conj. base
  8. Sep 27, 2009 #7


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    Staff: Mentor

  9. Sep 29, 2009 #8


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    I'm not sure how knowing how strong a conjugate base will be can help you here if you don't already know which is the stronger acid beforehand. That is circular reasoning IMO.

    You should remember that the pKa is the pH at which half of the acid is deprotonated. The higher the pKa, the higher the pH must be to deprotonate it. Thus, the compound with the higher pKa must be the weaker acid since it requires a stronger base to deprotonate it.
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