nevermind, found a good post..sorry!

if [OH] = 2.0 x 10^-4, then u can say pOH = 4 - log 2 = 3.7
you should try to memorize some simple logs, at least 1-5:
log 1 = 0
log 2 = 0.3
log 3 = 0.5
log 4 = 0.6
log 5 = 0.7
log 6 = 0.8
i just memorized it. its easy for 3-6, you just add 2 and put a decimal in front. log1 = 0 is also easy to memorize. hope that helps