Molecular Mass:
Molecular mass refers to the mass of a molecule. It is the sum of the atomic masses of all the atoms present in a molecule.

Formula Mass:
Formula mass, on the other hand, is the sum of the atomic masses of all the atoms in a chemical formula. It is used for ionic compounds and other substances that do not exist as discrete molecules.

Differences between Molecular Mass and Formula Mass:

1. Definition:
Molecular mass is the mass of a molecule, while formula mass is the mass of a formula unit.

2. Applicability:
Molecular mass is applicable for covalent compounds, where atoms are bonded together to form discrete molecules. Formula mass is applicable for ionic compounds, where ions are arranged in a lattice structure.

3. Calculation:
To calculate the molecular mass, we sum up the atomic masses of all the atoms in a molecule. However, to calculate the formula mass, we sum up the atomic masses of all the atoms in a chemical formula, considering the ratio of atoms in the formula unit.

4. Representation:
Molecular mass is represented in atomic mass units (u) or grams per mole (g/mol). Formula mass is also represented in atomic mass units (u) or grams per mole (g/mol).

5. Examples:
For example, in water (H2O), the molecular mass would be the sum of the atomic masses of two hydrogen atoms and one oxygen atom. In this case, the molecular mass would be approximately 18.015 g/mol.
For sodium chloride (NaCl), the formula mass would be the sum of the atomic masses of one sodium ion and one chloride ion. In this case, the formula mass would be approximately 58.44 g/mol.

Conclusion:
In summary, molecular mass is the mass of a molecule, while formula mass is the mass of a formula unit. Molecular mass is used for covalent compounds, whereas formula mass is used for ionic compounds. The calculation of these masses involves summing up the atomic masses of the atoms involved, but the approach differs based on the type of compound.