To determine the concentration of the hyroxide ion in the aqueous solution given, we need a relation between the hydronium ion concentration and the hydroxide ion concentration. For this case, we use the Kw or the water autoionization constant. It is a value which represents the self ionization of water when pure or in an aqueous solution. It ionizes into H+ ions and OH ions. Kw is equal to 1x10^-14 and is the product of the hydronium ion concentration and the hydroxide ion concentration. From this definition, we can calculate the concentration of the hydroxide ions as follows:
Kw = [H+][OH-]
1x10^-14 = 3.50 x10^-4 [OH-]
[OH-] = 2.857x10^-11 M
