Molecular weight vs formula weight

The mass of one molecule, computed from its formula — with the caveat that registry values describe the free base, while the material in a vial is a salt with residual water.

Molecular weight is the mass of one molecule of a compound, calculated by summing the atomic masses in its molecular formula; formula weight is the same arithmetic applied to whatever formula unit is stated, salt included. For a peptide the values are computed from the sequence, and registry databases publish them alongside the structure.

The caveat that rarely gets printed is which form the number describes. Registry values — including the formulas and weights shown on this site's research pages, which come from each compound's PubChem record — describe the free base. The powder in a vial is that molecule plus counterion plus residual moisture, so the registry weight understates the mass of powder that contains one mole of compound.

There is also a smaller technical distinction between average mass, computed from standard atomic weights, and monoisotopic mass, computed from the most abundant isotopes — mass spectrometry works with the latter, catalogue listings with the former. The values differ by a few daltons for a peptide-sized molecule, which is why an observed mass should be compared against the matching kind of theoretical value.

For research purposes only. Not for human consumption, diagnosis, treatment, or prevention of any condition. Nothing on this page is medical advice.