Calculate Ka for C5H5NH+?

Calculate Ka for the pyridium ion, C5H5NH+, at 25 degrees Celsius.

I think the only thing you need to know is that the Kb value for C5H5N is 1.8e-9

I made this equation, which I think is right, but I’m really not sure, but now I don’t know what to do from here: http://goo.gl/TDya9

2 Answers

  • It is the pyridinium ion. Pyridium is a brand name of a drug, phenazopyriine.

    For a weak acid and its conjugate base, Ka x Kb = Kw

    Ka = Kw / Kb = 1.00×10^-14 / 1.8×10^-9 = 5.6×10^-6

  • If you know the Kb, you can calculate Ka from the relationship KaKb = Kw = 1×10^-14

    Ka = Kw/Kb

    Ka = 1×10^-14/1×1.8×10^-9

    Ka = 5.6×10^-6

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