is the sum of the atomic weights of a molecule’s constituent atoms. The molecular weight of a chemical compound is often expressed in terms of "average molecular weight," i.e., the average weight of all the molecules in the compound. In describing a hydrocarbon, average molecular weight is commonly defined in terms of the compound’s boiling point or distillation range, since these characteristics normally reflect the size, or heaviness, of the average molecule.

Also see distillation test.