For the reaction:
4 P(s) + 5 O2(g) → 2 P2O5(s)
How many moles of P are required to react completely with 1.00 mole of O2?
How many moles of P2O5 are produced from the complete reaction of 1.00 mole of O2?
How many moles of O2 are required to produce 0.80 moles of P2O5?