Difference between Molecular mass and Formula mass.

Molecular Mass:
Molecular mass applies to covalent (molecular) compounds such as H₂O, CO₂, and CH₄. It is the sum of atomic masses of all atoms in one molecule.

Formula Mass:
Formula mass applies to ionic compounds such as NaCl, MgO, and AlCl₃. It is the sum of atomic masses of all atoms in one formula unit of a substance.

Key Difference:
For covalent compounds, both terms give the same value, but for ionic compounds, only formula mass is the correct term.