Say that this is the concentration of OH- ions in solution. All you have to do is to get the negative log of the [OH-] to get the pOH. After getting the pOH, get the pH of the solution.
pOH = -log[OH-]
= -log(1.0 * 10^-5)
= 5.00
This is actually the pOH value, not the pH. To get the pH, just subtract the pOH from 14.
pH = 14 - pOH
= 14.00 - 5.00
= 9.00
The value 2.5 x 10^-11 M is the concentration of OH- ions in solution. To get the pH, get the pOH first.
pOH = -log[OH-]
= -log(2.5 x 10^-11)
= 10.6
Then, subtract this from 14 to get the pH. The pH is 3.40.
Note: When you determine the pOH or pH of the sample, always make sure you enter -log in the calculator so you'll get a positive result.