He hydroxide ion concentration in household ammonia is 3.2 * 10-3 m at 25 °c. what is the concentration of hydronium ions in the solution

when Kw = [OH-][H3O+],

and when Kw is the ionic constant for water at 25°C = 1 x 10^-14

and we have [OH-] = 3.2 x 10^-3

this the concentration at equilibruim.

so by substitution:

1 x 10^-14 = 3.2 x 10^-3 * [H3O+].

∴ [H3O] = (1 x 10^-14) / (3.2 x 10^-3)

= 3.125 x 10^-12 m.

so, the answer is: concentration of hydronium ions is = 3.125·10⁻¹² M