pure water is a neutral solution where the hydrogen ion concentration is equal the hydroxide ion concentration.
[H⁺] = [OH⁻]
kw is ionic constant for dissociation of water, therefore
kw = [H⁺][OH⁻]
however since [H⁺] = [OH⁻]
kw = [H⁺]²
since pH is known, [H⁺] can be calculated
pH = -log[H⁺]
[H⁺] = antilog(-6.630)
[H⁺] = 2.34 x 10⁻⁷ M
kw = (2.34 x 10⁻⁷)²
kw = 5.5 x 10⁻¹⁴
