The concentration of hydroxide ion in an aqueous solution at 25°C is 1.1 x 10-4 M. What is the concentration of the hydronium ion?

In an aqueous solution, the the concentration of hydroxide ion and the concentration of hydronium ion always exist in equilibrium due to auto-ionization of water (K_W) that is the equilibrium constant.

Water dissociates and exists in equilibrium as $2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\leftrightarrow {\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

The autoionization of water (K_W) is the product of H₃O⁺ ion concentration and OH^- ion concentration.

${\mathrm{K}}_{\mathrm{w}}=\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]·\left[{\mathrm{OH}}^{-}\right]$

At 25^oC, the value of K_W = 1 x 10^-14.