Exact Molecular Mass versus Molecular Weight

In the discussion in the last section, the term exact mass was introduced. Again this is the mass of a molecule calculated with only the most abundant isotopes present; these are usually the lightest isotopes. The molecular weights given in the Handbook of Chemistry and Physics are different; they are the average molecular weights for all the molecules with different isotopic compositions present in a compound, each weighted for the natural abundance of the respective isotope or isotopes. For example the molecular weight of propane is 44.0956 g/mol and for acetaldehyde, 44.0526 g/mol. The exact molecular masses for the molecules with the lightest isotopes are 44.0626 amu and 44.0262 amu, respectively. Note, we use different units for molecular weight than we use for molecular mass.

Next section: Elements With More Abundant Heavy Isotopes

Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.