Relative formula mass, Mr, is determined by multiplying the subscript of each element in the formula by its relative atomic mass, Ar on the periodic table. The results for each element are added together to calculate the relative formula mass. The unified mass unit or dalton, u or Da, is applied to the relative formula and atomic masses, though they actually have no unit.
Example: What is the relative formula mass for sodium sulfate, Na2SO4 using whole numbers for the Ar.
Mr Na2SO4=(2×23 u Na)+(1×32 u S)+(4×16 u O)=142 u Na2SO4
When working with moles, the relative atomic and formula masses are numerically equal the molar mass. Molar mass is the mass in grams of one mole of a substance. So the mass of one mole of sodium, 23 u, would become 23 g/mol sodium, and the mass of one mole of sodium sulfate, 142 u, would be 142 g/mol.
