Atomic Mass of Hydrogen = 1u. There are 4 atoms of Hydrogen. 1u ✕ 4 is equal to 4u. Atomic Mass of Carbon is 12u. There is only one carbon atom. Hence the molecular mass of CH4 is 16u, which is 16gm/mol. The molecular mass (m) is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element.