posted by ben .
What is the concentration (in M) of hydroxide ions in a solution at 25°C with pH = 4.282?
I would do it this way.
pH + pOH = pKw = 14
Solve for pOH.
Then pOH = -log(OH^-) and solve for OH^-
You could convert pH to H^+, then
(H^+)(OH^-) = 1 x 10^-14
and solve for OH^-