The formula mass (formula weight) of a molecule is the sum of the atomic weights of the atoms in its empirical formula.
The molecular mass (molecular weight) of a molecule is its average mass as calculated by adding together the atomic weights of the atoms in the molecular formula.
So, since the definitions differ according to whether you are using the empirical formula or molecular formula for a molecule, it's a good idea to understand the distinction between them.
The molecular formula indicates the type and number of atoms in a molecule. The molecular formula of glucose is C6H12O6, which indicates that one molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.
The empirical formula is also known as the simplest formula. It is used to indicate the mole ratio of elements present in a compound. The empirical formula of glucose would be CH2O.
The formula mass and molecular mass of water (H2O) are one and the same, while the formula and molecular mass of glucose are different from each other.