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