Help with finding simplest formula!?

Question

11.15 grams of pure iron are allowed to react with oxygen to form an oxide, FexOy. If the product weighs 14.3g, what would be the simplest formula of the compound?

Answer 1

Find the mass of oxygen:

oxygen mass = 14.3 - 11.15 = 3.15 g

Find the moles of Fe and of O, dividing each mass by the Atomics mass of the element:

Fe: 11.15/56 = 0.2 mol Fe
O: 3.15/16 = 0.2 mol O

So 1 mol of Fe combines with 1 mol of O and the simplest formula is: FeO