What's the difference between a formula unit and a molecule?

Hint :We know that the molecular mass is a term for the average relative mass of a single molecule and formula mass is a term used for the sum of the atomic mass of constituent’s atoms. Molecular mass is the sum of the atomic masses of the atoms in a molecule means it gives the mass of a molecule.

Complete Step By Step Answer:
A molecule is composed of two or more elements that are covalently bonded and a formula unit indicates the lowest whole number ratio of ions in an ionic compound.
Also, molecular mass is also known as molecular weight and it actually gives the mass of a molecule relative to that of a carbon twelve atom which has a mass of twelve units. Formula mass of a substance is actually the sum of atomic masses of constituents atoms in an ionic compound or we can say that the sum of atomic masses of the ions present in the formula unit of an ionic compound.
Molecular mass and formula mass are somewhat the same terms but we use it differently because molecules are actually covalent compounds means they have only nonmetals and an ionic compound cannot be a molecule therefore we just simply say formula mass when we are taking ionic compounds.
The molecule is composed of two or more elements that are covalently bonded. It is the smallest particle of a covalent substance that has all of the properties of that substance, and it is the smallest particle of that substance that can participate in a chemical reaction. Meanwhile a formula unit indicates the lowest whole number ratio of ions in an ionic compound.

Note :
Molecular mass is for covalent compounds means compounds that contain only non-metals while formula mass is for ionic compounds means compounds that contain metals and non-metals because the positively and negatively charged ions in an ionic compound are arranged in a crystal lattice; there is no discrete particle comparable to a molecule.