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    Ethanol has a somewhat electron-donating alkylgroup attached to it, which should make its conjugate acid a WEAKER acid than H3O+... Anyway, R-COH2+ has a lower pKa than H3O+, which seems utterly illogical to me.

    Any help greatly appreciated!!
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    Hmm what are the values you have? And in what medium?
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    the medium should be water... it is not explicitly stated anywhere. the exact values are a pKa of -1, 74 for hydronium ion (H3O+) and -5 for CH3CH2OH2+ (protonated ethanol).
    This is soo illogical... hydronium should certainly be a stronger acid, given it does NOT have any electron donating groups attached to the oxygen.
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    Slightly surprised that there is such a difference...to be honest I'm not sure you can really trust pKa values this much when they go negative when they're in water, anyway.
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    (Original post by Bio_Science)
    the medium should be water... it is not explicitly stated anywhere. the exact values are a pKa of -1, 74 for hydronium ion (H3O+) and -5 for CH3CH2OH2+ (protonated ethanol).
    This is soo illogical... hydronium should certainly be a stronger acid, given it does NOT have any electron donating groups attached to the oxygen.
    These are the pKa values of the conjugate acids. The data is really more relevant to the basicity of the base from which they came.

    It is also more to do with the definition of ka and kb

    from the weak acid equilibrium:

    HA <==> H+ + A-

    ka = [H+][A-]/[HA] where all species are aqueous.

    For the conjugate base:

    A- + H2O <==> HA + OH-

    kb = [HA][OH-]/[A-]

    Hence ka x kb = [H+][OH-] = 1 x 10-14

    and pKa + pKb = 14

    ------------------------------------------------------

    For ethanol:

    C2H5OH + H2O <==> C2H5OH2+ + OH-

    pKb = 19

    n other words ethanol is a very weak base

    And as pKa + pKb = 14 then the pKa of the conjugate acid is -5.

    Water also produces some OH - ions

    H2O + H2O <==> H3O+ + OH-

    Now as the [OH-] in water is 1 x 10-7 that would give a kb value from the equation as:

    kb = [H3O+][OH-] as water is not included in the equilibrium constant definition.

    therefore kb = 1 x 10-14

    and pKb = 14

    And if pkb = 14 and pka + pkb = 14, it follows that pka = 0

    Now I'm not sure where the value for pKa of the hydronium ion comes from (given as -1.74)....
 
 
 
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