Sulfurous and Sulfuric Acid? Maybe they sound similar in the names but they have different chemical formulas. The chemical formula of Sulfurous Acid is H2SO3, while the formula for Sulfuric Acid is H2SO4. These two are composed of same chemical elements, however in different ratios as you can see in their formulas. Sulfuric Acid is stronger than sulfurous acid. H2SO3 has a higher pKa value because it is weaker.