In the same way, using the relative atomic masses given in the Mendeleev table, we can calculate the relative molecular mass of a molecule of any substance whose formula is known.Conclusion: The relative atomic mass of any atom of a chemical element has already been calculated by scientists and recorded in the corresponding cells of the periodic system of Dmitri Ivanovich Mendeleev.The molecular mass of substances is calculated based on the relative atomic masses of the atoms that make up the molecule.