A quantity of particles, atoms, or molecules of any substance that is equal to Avogadro's number is called one mole of that substance. Avogadro's number, a constant, is an immense number equal to 602,000,000,000,000,000,000,000, or 6.02x1023 particles of any material. Therefore, one mole of a substance is equal to 6.02x 1023 particles. Molar quantities, then, are amounts of chemical substances expressed as moles. For example, one mole of salt is Avogadro's number of salt molecules, two moles of hydrogen ions equals two times Avogadro's number of protons.

Molar quantities, by definition have known numbers of particles. Such quantities, however, may have very different weights. For example, one mole of water and one mole of carbon dioxide each have the same number of molecules, but the mole of water weighs 18 g, while the mole of carbon dioxide weighs about 48 g. The difference in mass occurs because one mole of...