Determine the mass fraction and percentage of iron and oxygen in iron oxide.

Iron has several oxides, their mass fractions are not the same, but they are found similarly. I will disassemble iron oxide with valence II
Mass fraction (w) is found using the periodic table.
For this we find the molecular weight of iron oxide (FeO). It is equal to 72.
Next, we find the atomic mass of each element and multiply by the number of atoms of that element. For iron it is 56, and for oxygen it is 16. In our case, there is no need to multiply by anything, since the atoms of Fe and O are one each.
But if we took another iron oxide, for example, with a valence of III, then we must multiply there.
Next, we are looking for w, for this we divide the atomic mass by molar.
w (Fe) = 56/72 = 0.778 (rounded to thousandths) as a percentage this is 77.8%
w (O) = 16/72 = 0.222 (also rounded up) as a percentage this is 22.2%