The mol is one of the bas units in the SI system of units and is a unit of measurement used to express amounts of a substance, defined as an amount of a substance that contains as many elementary entities (e.g.,atoms, molecules, ions, electrons) as there are atoms in 12 grams of Carbon - 12, the isotope of carbon with atomic weight 12. This corresponds to a value ofelementary entities of the substance.
The mole is widely used instead of mass or volume, as a convenient way to express the amounts of the sbstances in chemical reaction chemical reactions and may also be used to express the number of atoms, ions, or other elementary entities in some sample. The concentration of a solution is commonly expressed by its molarity, the number of moles of the dissolved substance per liter of solution.
The number of molecules in a mole (known as Avogadro's number) is defined so that the mass of one mole of a substance, expressed in grams, is exactly equal to the substance's mean molecular weight. For example, the mean molecular weight of natural water is about 18.015, so one mole of water is about 18.015 grams. This property considerably simplifies many chemical and physical computations.