What is the difference between molecular weight and formula weight?
It is imperfect to define molecular weight for the substance having no molecular entity as in the case of purely ionic compound like NaCl. So Formula weight is more perfect for it.
Some compounds do not exist as individual molecules. For example, there is no such thing as a molecule of sodium chloride. That's why it is appropriate to use the term formula weight for this type of compound. However, some compounds do exist as individual molecules. For example, water,carbon-dioxide etc. So it is appropriate to use the term molecular weight for it.