Acompound contains 69.9% iron and 30.1% oxygen. what is its empirical formula?

The easiest way to do this is to assume that you have 100g of compound. Then you will have 69.9g of iron and 30.1g of oxygen.

You will need to find how many moles there are of each. Divide these masses by the molar mass of each element (for oxygen, just use the molar mass of O, not O2, as the calculation is easier with atoms than dioxygen):

69.9g / 55.845g/mol = 1.25mol Fe
30.1g / 15.999g/mol = 1.88mol O

Next, find the ratio of these 2 molar amounts. If we have 1 mole of Fe, we will have 1.88 / 1.25 = 1.50 mol O. Make everything whole numbers now: 1.50 is half of 3, so multiply both by 2. We get 2 moles of Fe to every 3 moles of O, so the empirical formula is Fe2O3.

It shows that the experiment is designed properly.