the.com/molecular weight

means The total mass of a molecule, calculated by adding up the atomic weights of all the atoms in it.

from 'Molecule' traces back to Latin moles ('mass, bulk') via French molécule — a diminutive meaning something like 'small mass.' 'Weight' is just old plain English, from Old English wiht/gewiht, tied to the verb 'to weigh.' The phrase itself is a product of 19th-century chemistry, once atomic theory let scientists like Dalton and later Avogadro's followers actually tally up atoms and assign them comparative masses — turning an abstract 'small mass' into something you could put a number on.