The mole (abbreviation: mol) is the base unit of amount of substance in the SI system of units. One mole equals the amount of substance that contains as many elementary units as there are atoms in 0.012 kilogram of carbon-12. The elementary units may be atoms, molecules, ions, radicals, electrons, etc., and must be specified. 1 mole of a compound has a mass equal to its relative molecular mass expressed in grams. The mole replaced the gram-atom.
The molarity of a solution is its concentration in moles per liter; a solution whose molarity is 1 is called molar.
The number of elementary units in 1 mole is called the Avogadro constant (formerly known as Avogadro's number).
A gram-atom is the quantity of an element the mass of which, in grams, is equal to its relative atomic mass. It has been superceded by the SI unit, the mole. For example, one gram-atom of hydrogen (H, r.a.m. = 1) is 1 gram.
Molar volume is the volume occupied by one mole of a substance an has the value 22.414 liters. It is approximately the same for all gases at standard temperature and pressure.