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 pK_a value because it is weaker.