Calculating the pH of a solution...?

Question

with a hydroxide concentration of 3.7 x 10 ^-8(ten to the negative eighth). I got a pH of 7.4, but my friend said it was 6.6. Which one of us is right? And if it is 6.6, how did you get that?
Thanks

Answer 1

Your friend was correct.
As you said, the hydroxide concentration is 3.7x10^-8 M. that means [OH-] = 3.7x10^-8 M.
pOH = - log([OH-]), but:
pH = 14 - pOH = 14 + log([OH-]) = - log ([H+])