In case of ionic compounds we have crystal lattice composed of ions. And there is a never ending pattern of ions and that made it difficult to calculate the molecular mass. Thus we used a simple method by taking the ratio of the ions and then adding their masses, like in case of NaCl ; 23 of Na + 35.5 of Cl = 58.8 amu formula mass of NaCl.
In case of molecular mass, that is simply taken in case of compounds that are confined to a limit as in case of CO2, we simply add the mass of Carbon 12+ 16×2 of oxygen = 44 amu is the molecular mass of CO2.
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