Iron is an element ... amounts or ratios like they are in a compound, e.g. two hydrogen atoms for each oxygen atom in water. They can be in any combination, e.g. for a mixture of sand and water ...

of~an~element} = \frac{mass~of~an~element~in~the~compound}{relative~formula~mass~M_r} \times 100\) There are six atoms of hydrogen in the compound, so the mass of hydrogen is 6 × 1 = 6.