molar volume, the volume occupied by a mole of a substance at STP. According to Avogadro's law, at a given temperature and pressure a given volume of any gas contains the same number of molecules. At STP 1 mole of gas occupies 22.414 liters. This volume is what is usually meant by the molar volume, although one can also speak of molar volumes of substances that are not gaseous at STP. Since at STP water occupies very nearly 1 cc per gram, and since 1 mole of water molecules weighs very nearly 18 grams, the molar volume of water is about 18 cc. The molar volume of osmium, one of the densest metals, is only about 8.4 cc; that of beryllium is only about 4.86 cc.

gram-molecular volume, the volume occupied by one gram-molecular weight of any substance at STP. See molar volume.