Question

certain solution has [OH-] = 4 x 10-3. Calculate the concentration of [H3O+] and the pH of the solution

Answer 1

Kw = 1 x 10⁻¹⁴

Kw = [ H₃O⁺] [ OH⁻]   

1 x 10⁻¹⁴ = [ H₃O⁺] * 4 x 10⁻³

[H₃O⁺] = 1 x 10⁻¹⁴ / 4 x 10⁻³

[H₃O⁺] = 2.5 x 10⁻¹² M

hope this helps!