The difference between Molecular Mass and Formula Unit Mass is that "Molecular Mass" term is used for covalent compounds and "Formula Unit Mass" term is used for ionic compounds.
HNO3 is a covalent compound in its purest form but when we add water in it, HNO3 slipts into H+ and NO3 - ions and further H+ ions attaches itself to H2O to give H30 which is an hydronium ion which is ionic and CaCl2 is an ionic compound.
HNO3 is a covalent compound in its purest form but when we add water in it, HNO3 slipts into H+ and NO3 - ions and further H+ ions attaches itself to H2O to give H30 which is an hydronium ion which is ionic and CaCl2 is an ionic compound.