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Sulfur forms sulfur oxoacids, some of which cannot be isolated and are only known through the salts. Sulfur dioxide and sulfites () are related to the unstable sulfurous acid (H2SO3). Sulfur trioxide and sulfates () are related to sulfuric acid (H2SO4). Sulfuric acid and SO3 combine to give oleum, a solution of pyrosulfuric acid (H2S2O7) in sulfuric acid.