Molecular weight (Chem.), the weight of a molecule of any gas or vapor as compared with the hydrogen atom having weight of 1 as a standard; the sum of the atomic weights of the constituents of a molecule; thus, the molecular weight of water (H2O) is 18. For more precise measurements, the weight of the carbon isotope carbon-12 is used as the standard, that isotope having the value of 12.000. In this systen, now used almost universally, the hydrogen atom has a weight of 1.0079.