The acid anhydrides of H3PO3 (phosphorous acid) and H3PO4 (phosphoric acid) are formed by removing water (H2O) from the respective acids.
The acid anhydride of H3PO3 is P2O5, known as phosphorus pentoxide. When P2O5 reacts with water, it forms phosphorous acid (H3PO3).
P2O5 (phosphorus pentoxide) + H2O → 2H3PO3 (phosphorous acid)
The acid anhydride of H3PO4 is P4O10, known as tetraphosphorus decoxide. When P4O10 reacts with water, it forms phosphoric acid (H3PO4).
P4O10 (tetraphosphorus decoxide) + 6H2O → 4H3PO4 (phosphoric acid)
So, the acid anhydride of H3PO3 is P2O5, and the acid anhydride of H3PO4 is P4O10.