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Conjugate acids and bases

  1. Jun 8, 2005 #1
    Uggh finally did all the chem problems and studied for the upcoming final but there are 2 questions left unanswered. MAybe someone can help me out with this:

    If given a lists of acids, how would one know which 1 is the weakest conjugate base? For example, HF, HNO_2, H_2CO_3, H_3BO_3, HCl, which 1 would be it? I didn't know to solve for it.

    Also, a conjudate acid-base pair consists of two substances that:

    -differ by 1 proton
    -neutralize each other
    -have equal number of protons
    -satisfy oth definition

    BY eliminationg all the choices, i concluded that its none but i'm still not sure.
  2. jcsd
  3. Jun 9, 2005 #2


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    Staff: Mentor

    The stronger the acid the weaker the conjugate base. It works both sides - the stronger the base, the weaker the conujgate acid.

    For every answer eliminated, explain why, so that we can point you to your error.
  4. Jun 9, 2005 #3
    I think it's none because none of the options with the definition of an acid-base pair. They obviously don't neutralize each other and the number of protons in this case is irrelevant. That's what i think anyway. Am i right?
  5. Jun 9, 2005 #4
    i dunno, i would imagine that HCl is the strongest acid and therefore Cl- would be the weakest conjugate base.
  6. Jun 9, 2005 #5


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    Staff: Mentor

    Write down any pair of acid and conjugate base.
  7. Jun 10, 2005 #6

    Why is it so? can plz give me the reason that why the conjugate base of an strong acid is weak?

    When we define a strong acid (or base), we say that it is an acid (or base) which ionizes to a higher extent in acquous solution. What is the reason that it does so? Is the polarity of its bonds that causes the ionization?
  8. Jun 10, 2005 #7


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    Staff: Mentor

    acid dissociation:

    [tex]HA \leftrightarrow H^+ + A^-[/tex]

    acid dissociation constant:

    [tex]Ka = \frac {[H^+][A^-]}{[HA]}[/tex]

    conjugated base dissociation:

    [tex]A^- + H_2O \leftrightarrow HA + OH^-[/tex]

    conjugated base dissociation constant:

    [tex]Kb = \frac {[HA][OH^-]}{[A^-]}[/tex]

    water dissociation:

    [tex]H_2O \leftrightarrow H^+ + OH^-[/tex]

    water dissociation constant:

    [tex]Kw = [H^+][OH^-][/tex]

    and now Ka*Kb:

    [tex]Ka Kb = \frac {[H^+][A^-]}{[HA]} \frac {[HA][OH^-]}{[A^-]} =
    [H^+][OH^-] = Kw[/tex]

    In other words (or symbols):

    [tex]pKa + pKb = pKw[/tex]

    The stronger the acid, the weaker the conjugated base.
  9. Jun 11, 2005 #8
    Thanks! this answers my first question, now what about the second one: Is it the polarity of bonds that characterizes a strong acid or base or something else? what is that?

    Dooh! please forgive me if I'm HACKING your thread... :biggrin:
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