Gram-Molecule

GRAM-MOLECULE, gram-mole, the number of grams of a given substance equal to its molecular weight. Thus, the molecular weight of oxygen is 32; therefore, if one takes 32 g of oxygen, this will be the gram-molecule of oxygen. The molecular weight of carbon dioxide is 44; therefore, the gram-molecule of carbon dioxide equals 44 g of this gas. The gram-molecule of any substance in an ideally gaseous state under equal conditions of temperature and pressure occupies the same volume, namely—under normal conditions—22.41 liters (Avogadro-Gerard law). Thus, the volume of 32 g of oxygen, 2.016 g of hydrogen, 44 g of carbon dioxide (measured, of course, under the same conditions of temperature and pressure) are identical. The concept of the gram-molecule allows for very convenient stoichiometric calculations and, in particular, for converting the weight of a gas to its volume.