gram-molecular weight, amount of a molecular substance whose weight, in grams, is numerically equal to the molecular weight of that substance. For example, one gram-molecular weight of molecular oxygen, O₂ (molecular weight approximately 32), is 32 grams, and one gram-molecular weight of water, H₂O (molecular weight approximately 18) is 18 grams. The term mole is often used in place of gram-molecular weight. See gram-atomic weight.