At 25 °c, what is the hydroxide ion concentration, [oh–], in an aqueous solution with a hydrogen ion concentration of [h ] = 4.2 × 10–9 m?

[oh-] = m

The equation for [OH-] concentration to [H+] concentration is [OH-] x [H+] = 1.00 x 10^-14. Next, we can plug in our [H+] concentration and get an equation of [OH-] x 4.2 x 10^-9 = 1.00 x 10^-14. Finally, we can divide both sides by 4.2 x 10^-9 and get our final answer of [OH-] = 2.4 x 10^-6 M.

Hope this helps!

Dmight be the correct answer