||Formula weight and molecular weight have slightly different definitions, though for many substances, the two measurements are the same (and we've been told the terms are used interchangeably by many chemists).|
Molecular weight is the sum of the atomic weights of atoms in a molecule's molecular formula.
Formula weight is the sum of the atomic weights of the atoms in a molecule's empirical formula.
Molecular formula is a notation that indicates the type and number of atoms in a molecule. The molecular formula of glucose is C6H12O6, which indicates that a molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.
Empirical formulas show which elements are present in a compound, with their mole ratios indicated as subscripts. For example, the empirical formula of glucose is CH2O, which means that for every mole of carbon in the compound, there are 2 moles of hydrogen and one mole of oxygen.
However, for a substance like Mescaline with a molecular formula of C11H17NO3, the empirical formula is exactly the same, making the formula weight and molecular weight identical.
The above definitions are from :
Hope that helps.