The balanced equation for the ionization of sulfuric acid in water is:

H2SO4 + 2H2O ⇌ 2H3O+ + SO4^2-

In this equation, one molecule of sulfuric acid (H2SO4) reacts with two molecules of water (H2O) to form two hydronium ions (H3O+) and one sulfate ion (SO4^2-).