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Disulfuric acid has the molecular formula of H2S2O7 where the S-atom is in the＋5 oxidation state. It is a major constituent of fuming sulfuric acid (also known as “oleum”) described by the formula ySO3·H2O where y is the total molar sulfur trioxide content. They can also be described by the formula: H2SO4·xSO3, which are also minor constituents of liquid anhydrous sulfuric acid due to the equilibria:
H2O＋SO3→H2SO4＋SO3→H2S2O7
Thus, the disulfuric acid is prepared by reacting excess SO3 with sulfuric acid:
H2SO4＋SO3→H2S2O7
Disulfuric acid is a strong acid and protonates sulfuric acid in the (anhydrous) sulfuric acid solvent system. An alternative IUPAC name is (sulfo-oxy)sulfonic acid.

ChEBI: Disulfuric acid is a sulfur oxoacid.