Question

The pH of a solution is 10. What is its OH- concentration?

Answer 1

"p" just means "-log" of something. So pH just means -log(H). H is the hydrogen concentration. Of course, you want the OH concentration, so what you can do is take subtract 10 from 14 to get 4, which is the pOH. Now all you have to do is 10^-(-pOH) to undo the negative and logarithm and that will tell you your OH concentration.

This works because Kw=Ka*Kb=10^-14

I can explain this more if you'd like!