Mass of molecule = Weight of molecule in Atomic Mass Units (AMU)
Gram Molecular Mass = Weight of one mole of molecules
To measure the mass of molecules, the measure used is the Atomic Mass Unit, or AMU. An atomic mass unit is precisely
The gram molecular mass is the mass of one mole of molecules. A mole is defined as "the amount of a chemical substance that contains as many representative particles, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of carbon-12" (wikipedia/wiki/Mole_(unit). This means that carbon-12 has a mole of one, and a gram molecular mass of 12 g, by definition.
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