Asked by chem101

The pKb of ammonia, NH3, is 4.74 at 25°C.  Which of the following is the Ka of the ammonium ion, NH4+?

not sure what to do, this is what i have i guess:
kb = -log 4.74
-----
pKa + pKb = 14
pKa = 9.26

ka = -log 4.26
= -0.676
-----

Ka = [NH4+][OH-]/ [NH3]

Answered by DrBob222
The easy way.
pKa + pKb = pKw = 14
You have pKa of 9.26 which is right but you took the -log of 4.26 which isn't right.
pKa = 9.26 = -log Ka
Ka = ?
Answered by chem101
thank you silly of me and to think i spent so long on that on a silly error.
Answered by DrBob222
Ahhh---but we do that sometimes. It is a GREAT learning experience. We aren't likely to do it twice.:-)
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