Molecular Mass

→ It is the sum of atomic masses of all the atoms in a molecule of that substance.

Example: Molecular mass of H2O = 2×Atomic mass of Hydrogen + 1×Atomic mass of Oxygen

So, Molecular mass of H2O = 2×1 + 1×16 = 18 u

Formula Unit Mass

→ It is the sum of atomic mass of ions and atoms present in formula for a compound.

Example: In NaCl,

Na = 23 a.m.u.

Cl = 35.5 a.m.u.

So, Formula unit mass = 1×23

+ 1×35.5 = 58.5 u