In mass spectroscopy, the mass-to-charge ratio (symbols: m/z, m/e) of a cation is equal to the mass of the cation divided by its charge.

Since the charge of cation formed in the mass spectrometer is almost always +1, the mass-to-charge ratio of a cation is usually equal to the mass of the cation.

The mass of the molecular ion is equal to the molecular weight of the compound.  Thus, the mass-to-charge ratio of the molecular ion is equal to the molecular weight of the compound.