[newbiesig]

I'm trying to relearn conjugate base pairs and I'm stumped on one problem. Such as given H2O Pka=15.7 with OH-; and NH3 pKA =36 with NH2-. Which is a stronger conjugate base OH- or NH2-. While going through the problem I'm wondering why for ammonia (NH3), pKA + pkb does not equal 14. Why is its pkb 4.7 while its pKa is 36. Also answering my other question would be helpful thank you.

[Borek]

pKa + pKb = pKw

pKw is a property of WATER so this formula works only for water solutions. Bases and acids stronger than water itself react with water first, so they never exhibit their full strength in water (so called leveling effect).

[opsomath]

What Borek said. Also, do you know how to go about deciding which of two things are the stronger base?

[magician4]

In addition:

While going through the problem I'm wondering why for ammonia (NH3), pKA + pkb does not equal 14.

it does add up !
you just need to be clear about which of the two conjugated acid/base pairs you"re talking :

NH₄⁺ /NH₃  pKa 9.25 / pKb 4.75

NH₃ / NH₂^- pKa 36 / pKb -50

regards

Ingo