Should I use ka or kb

They give us ka = 1.3 x 10^-5, and therefore kb = 7.9 x 10^-10 for an unknown Acid, HA

If I have the equation

A- + H20 => HA + OH-
.05 .................0......0
- x.................+x.....+x
(.05 -x).............x.......x

Do I set (x^2)/(.05-x) equal to Ka or Kb?


Thanks

To answer your question: Kb

Use Ka for Acid dissociation.

in equations like this u should see OH- is released or H+
if OH- released its base if H+ realsed its acid
so here u should equal to Kb