How it works
Because mass is additive, a molecule’s weight is the sum of the masses of its atoms, which is why it can be computed exactly from the formula. Two conventions are in use: the monoisotopic mass (each element’s most abundant isotope) and the average mass (weighted over natural isotope abundances).
The two figures differ, and mixing them is a common error. High-resolution mass spectrometry resolves the isotope pattern and is compared against the monoisotopic value; a lower-resolution or nominal figure is usually the average. A number should be read knowing which convention it follows.
Molecular weight is a property of the molecule, not of a lot. It does not change from batch to batch; what changes between lots is purity, which is measured separately.
Why it matters
Molecular weight is what makes identity checkable. ESI-MS measures a mass-to-charge ratio, and comparing the observed value against the weight computed from the formula is how identity is confirmed.
It is also a sanity check on the formula: an independently computed mass that disagrees with the stated weight flags an error in one of them — a check that has caught mistakes a name alone would pass.
Like the formula, it is necessary but not sufficient: isomers share a mass, so a matching weight confirms the mass but not the arrangement of atoms.
At Lineará
Each compound’s molecular weight is published on its compound page and, where applicable, on the certificate, in g/mol alongside the molecular formula.
The expected mass series a certificate’s identity check relies on is derived by construction and reconciled against an independently computed mass.