Not all substances are molecular?

Molecular mass is the sum of all the atoms in a molecule.

Ionic substances do not make molecules in the same way(the ions combine in billions of billions) so we simplify it down to the simplest statement of the ratio of the ions.

egs Covalent water has molecules H2O with molecular mass 2 x 1 + 16 = 18

Ionic NaCl has the simplest ionic unit 1 Na+ and 1 Cl- and formula mass 23 + 35.5 = 58.5

Ionic CaCl2 has the simplest ionic unit one Ca2+ and 2 Cl- so formula mass is 40 + 2 x 35.5 = 111

These terms are often used interchangeably but you should use the separate terms for covalent and ionic compounds.

NaCl is a molecule.

Molecular mass is the sum of the atomic masses of the atoms making in a molecule. E.g that of NaCl is 23 + 35.5 = 58.5 a.m.u. (atomic mass unit)

Formula mass for a molecule is the same as molecular formula only that it has no units.

Also, it is used for substances like ions and atoms.

Not all substances are molecules, we have atoms and ions also.

And formula mass expresses the mass of one mole of a substance ( without units)

Hope u get it.. molecular mass for molecules.

Formula mass for any substance be it molecules, atoms or ions.

NaCl, KCl, MgO are molecules.

Na, Ag, Ca are atoms.

H-, NO3- are ions.

Atomic mass is the mass of the atom, molecular mass is the mass of the molecules. In nature everything except noble gasses forms molecules, for example an Oxygen atom on the periodic table is O but in air you find Oxygen floating around as diatomic oxygen molecules O2.