58.5 amu is the formula mass of Nacl not the molecular mass?wy
We use the term formula mass with NaCl and not the molecular mass because NaCl is an ionic compound which exits in the form of Na+ and Cl- ions, and not as discrete molecules. Molecules have covalent bonding between the bonding atoms and are formed when the atoms share a given number of electrons. In NaCl, there is no sharing of electrons, rather the transfer of electrons is taking place. As the term molecular mass is used in reference to molecules only and not ions, therefore for NaCl we use the term formula mass.