Stoichiometry, Moles, and Molecular Weight

 The  molecular weight is the sum of the atomic weights of all the atoms in a molecule of the substance.  Hence, the average mass of a molecule is the molecular weight, also expressed as amu.  A formula weight is the sum of the atomic weights of all atoms in a formula unit of a compound, whether the compound is a molecule or not.  Molecular weight is accordingly a subset of formula weight -- as molecular weight does not really apply to ionic molecules (like NaCl, sodium chloride, ordinary table salt).