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⁺] = [OH⁻] = sqrt(1.47*10^-14) = 1.21*10^-7
The concentration of hydroxide ions in pure water is 1.21*10^-7.
