Sometimes the empirical formula and the molecular formula are the same for a molecule. That is, atomic mass of molecules. (Public Domain) via Commons Wikimedia. 4. Moreover, we can calculate molar mass of the compounds like NaCl or CuSO4 by adding the atomic masses of each atom. For example, CO2 or carbon dioxide is a compound. For more information contact us at [email protected] or check out our status page at https://status.libretexts.org. Molecular mass of a compound is the mass of one molecule. Here, the atomic weights of each atom in g/mol unit are added to get the molecular mass. The molar mass of any substance is the mass in grams of one mole of representative particles of that substance. The key difference between formula mass and molar mass is that, the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula while  molar mass is the mass in grams of 1 mol of substance. Plugging that in yields: Adding the molecular weights of each element together gives: Thus, the molecular weight of water is 18.02 amu. One of the ways this improvement comes about is through measuring the amount of material produced in a reaction. Formula mass and molar mass are two physical properties of molecules that show some difference between them. Molecular mass of a compound is defined as the mass of one molecule. Since the atomic masses of each element are known, it is easy to calculate the molecular mass rather than thinking of a 6.023 x 1023 of molecules. The answer has three significant figures because of the $$0.560 \: \text{mol}$$. For normal samples from earth with typical isotope composition, the atomic weight can be approximated by the standard atomic weight or the conventional atomic weight. “Review This Sample Problem to Find Molecular Mass of a Compound.” ThoughtCo, Available here. (Public Domain) via Commons Wikimedia2. 3. The mass of C atom                 = 12.0107amuThe mass of an H atom             = 1.0079 amuTherefore the formula mass  = (2 x 12.0107amu) + (5 x 1.0079 amu)                                                     = 29.0609 amu. For example, the molecular weight of oxygen is 16.00 amu. Examples of molecular weight computations: CO2, SO2. \: \text{g}\). They are very important concepts in chemistry. 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