The balanced equation for the reaction between tetraphosphorus decoxide (P4O10) and water (H2O) to produce phosphoric acid (H3PO4) is:

P4O10(s) + 6 H2O(l) → 4 H3PO4(aq)

In this reaction, one mole of tetraphosphorus decoxide reacts with six moles of water to produce four moles of phosphoric acid.