Fomula mass and molecular mass are two values that express the size of a molecule. Do you know the difference between formula mass and molecular mass?
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. The formula mass (formula weight) of glucose is 30 (either no units or else grams per mole), while the molecular mass (molecular weight) is 180.156 g/mol. Whenever you see a molecular formula where you can divide the subscripts by a whole number (usually 2 or 3), you know to expect the formula mass will be different.