The amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 g of carbon-12 (i.e., 6.023 × 1023). This involves the acceptance of two dictates--the scale of atomic masses and the magnitude of the gram. Both have been established by international agreement. Formerly, the connotation of “mole” was “gram molecular weight.” Current usage tends to apply the term “mole” to an amount containing Avogadro's number of whatever units are being considered. Thus, it is possible to have a mole of atoms, ions, radicals, electrons, or quanta. This usage makes unnecessary such terms as gram-atom, gram-formula weight, etc. All stoichiometry essentially is based on the evaluation of the number of moles of substance. The most common involves the measurement of mass. Thus 25.000 g of water will contain 25.000/18.015 moles of water, 25.000 g of sodium will contain 25.000/22.990 moles of sodium. The convenient measurements on gases are pressure, volume, and temperature. Use of the ideal gas law constant R allows direct calculation of the number of moles: n = PV/RT. T is the absolute temperature, R must be chosen in units appropriate for P, V, and T. The acceptance of Avogadro's law is inherent in this calculation; so too are approximations of the ideal gas.See Avogadro's Law.
