Remember that in water, the concentration of hydronium ions, or H⁺, will equal the concentration of hydroxide ions. That is [H⁺] = [OH⁻].
Remember that Kw = [H⁺][OH⁻] for water and if [H⁺] and [OH⁻] are both equal, we can easily solve for them by taking the square root of Kw.
[H⁺] = sqrt(0.12*10^-14) = 3.46*10^-8
(2 sig figs. Keep some extra so there will not be a rounding error.)
and remember that pH = -log[H⁺] = -log(3.46*10^-8) = 7.46
(The rules for sig figs are that if you take the log of a number with x significant figures, then the result should have x significant decimal places.)
The pH of water is 7.46
