Molecular and Formula Weight
Molecular Weights
For a molecular substance, the molecular weight is the average mass of a molecule of a substance relative to the mass of a carbon-12 atom. In other words, the molecular weight is the sum of the masses of the atoms in a molecular formula. For illustration we will determine molecular weight to four significant figures for C3H4Cl2O2 .
| 3 x atomic weight of C = 3 x 12.011 amu = 36.033 amu |
| 4 x atomic weight of H = 4 x 1.008 amu = 4.032 amu |
| 2 x atomic weight of Cl = 2 x 35.453 amu = 70.906 amu |
| 2 x atomic weight of O = 2 x 15.999 amu = 31.998 amu |
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| Molecular weight of C3H4Cl2O2 = 142.969 amu = 143.0 amu |
Formula Weights
For ionic compounds and other substances in which individual molecules do not exist, it is more appropriate to use the term formula weight. Formula weight is the average mass of a formula unit relative to that of a carbon-12 atom. The operation of determining a formula weight is identical to that of determining a molecular weight. As an example lets determine the formula weight of Al2(CrO4)3.
| 2 x atomic weight of Al = 2 x 26.982 amu = 53.964 amu |
| 3 x atomic weight of Cr = 3 x 51.996 amu = 155.988 amu |
| 12 x atomic weight of O = 12 x 15.999 amu = 191.988 amu |
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| Formula weight of Al2(CrO4)3 = 401.940 amu = 401.9 amu |