Final answer:
In a 2.0 x 10^-3 M solution of HNO3, the hydronium ion concentration is 2.0 x 10^-3 M and the hydroxide ion concentration is 5.0 x 10^-12 M.
Step-by-step explanation:
The concentration of hydronium ions ([H3O+]) in a 2.0 x 10-3 M solution of HNO3, a strong acid, is equal to the initial molarity of the acid since it completely dissociates in water. Thus, [H3O+] = 2.0 x 10-3 M.
The concentration of hydroxide ions ([OH-]) in the solution can then be found by using the ion product constant of water (Kw) at 25°C, which is 1.0 x 10-14. So [OH-] = Kw / [H3O+] = 1.0 x 10-14 / 2.0 x 10-3 M = 5.0 x 10-12 M.