In this problem you would need to determine the mass of each
element first. The mass of phosphorus is given which is 31 grams. The mass of
oxygen is the difference between 71 grams and 31 since we know that the compound
formed weighs 71 grams. Then determine the moles of each element. You would get
1.0009 moles for phosphorus and 2.5 moles for oxygen. You have to then multiply
the moles to the lowest number that would give a whole number for each element.
The empirical formula of the oxide is P2O5.
