Determine the relative molecular weight of phosphoric acid, the molecule of which consists of three hydrogen atoms

Determine the relative molecular weight of phosphoric acid, the molecule of which consists of three hydrogen atoms, one phosphorus atom and four oxygen atoms. What is the mass fraction of phosphorus in this compound?

Mr (H3PO4) = 3 * 1 + 31 + 4 * 16 = 98. To find the mass fraction of phosphorus, you need to multiply the atomic mass of phosphorus by the number of atoms, divide by Mr (H3PO4) and multiply by 100%: w% (P) = (31 * 1/98) * 100% = 31.63%.