How many moles of O2 are needed to make 71 moles of diphosphorus pentoxide?

Question

The equation is: 4P + 5O2 → 2P2O5

Answer 1

Use the stoichiometry of the balanced equation along with dimensional analysis to answer the question:

71 mols P₂O₅ x 5 mol O₂ / 2 mols P₂O₅ = 177.5 mols = 180 mols O₂ needed (2 sig. figs.)

Answer 2

71 mol 2P₂O₅ x (5 mol O₂ / 2 mol 2P₂O₅) = 177.5 mol O₂