Self-Ionization Constant of Water (Kw)

States that the product of hydrogen and hydroxide concentrations is fixed at a given temperature, so the two ions cannot vary independently.

$$K_w=[{\mathrm{H}}^{+}][\mathrm{O}{\mathrm{H}}^{-}]$$

Variables

Symbol	Meaning	Unit
$K_w$	Self-ionization constant of water (1.0 x 10^-14 at 25 C)	mol^2/L^2
$[{\mathrm{H}}^{+}]$	Hydrogen-ion concentration	mol/L
$[\mathrm{O}{\mathrm{H}}^{-}]$	Hydroxide-ion concentration	mol/L

Worked example

At 25 C, [H+] = [OH-] = 10^-7 mol/L, so Kw = (10^-7)(10^-7) = 10^-14.

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Part of The Blueprint of Water