phosphorus forms two oxides, which have the empirical formulae p2O3 and P2O5. what mass of phosphorus will combine with 1 mole of oxygen molecules (O2) to form P2O3?
You’re going to need 2/3 mol of phosphorus in order to have the reaction go to completion with 1 mol of oxygen, as for every 3 moles of oxygen, you will need 2 moles of phosphorus. Based on this, simply multiply the molar mass of phosphorus by 2/3, and you should get your answer.
30.97 g/mol * 2/3 mol = 20.65g