The acid anhydride of HNO2 (nitrous acid) is called nitrous anhydride or dinitrogen trioxide (N2O3). It is formed by the combination of two molecules of nitric oxide (NO) and one molecule of nitrogen dioxide (NO2):
2NO + NO2 → N2O3
The acid anhydride of H2SO4 (sulfuric acid) is called sulfur trioxide (SO3). It is formed by the removal of a water molecule from two molecules of sulfuric acid:
2H2SO4 → 2H2O + SO3
Both nitrous anhydride (N2O3) and sulfur trioxide (SO3) are highly reactive compounds and are typically handled with caution due to their reactivity with moisture in the air.