Stoichiometry, Moles, and Molecular Weight

For Fe₂(SO₄)₃
                    2 x Fe = 2 x 55p8 amu          = 111.6 amu
                    3 x S   = 3 x 32.1 amu           =  96.3 amu
                    3 x 4 x O = 12 x 16.0 amu    = 192.0 amu
                                                                    399.9 amu

Consider that there are about 1.1 x 10²³ formula units of NaCl in a teaspoon of pure table salt.  Chemists routinely work with such large numbers of molecules.  So, they adopted the mole concept as a way to deal with such large numbers of molecules or ions.