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(or mole), the number of grams of a simple or complex chemical compound equal to its molecular weight. Thus, if the molecular weights of nitrogen N2 and sulfuric acid H2SO4 are 28.0134 and 98.078. respectively, their gram molecules are equal to 28.0134 g and 98.078 g. For ease in computation, the value of the gram molecule is rounded off to the nearest gram. The number of molecules in one gram molecule of any substance is the same and is equal to Avogadro’s number, that is. 6.02252 х 1023. The gram molecule of any substance in the ideal gas state at standard conditions (a temperature of 0°C and a pressure of 101,325 newtons/m2 [760 mm Hg]) occupies approximately the same volume, which equals 22.4 liters. Based on this relation, the volume of any amount of gas of known molecular formula can be easily computed. Thus. 66 g of C02 (molecular weight 44) is 1.5 gram molecules, and consequently at standard conditions it occupies a volume of 22.4 liters х 1.5 = 33.6 liters. It is conventional also to speak of the gram molecule of a mixture: thus the gram molecule of air is taken to be equal to 29 g.
The concept of gram molecule, as well as of kilogram molecule and ton molecule, is widely used in various computations in chemistry, physics, and engineering.