Mole is a unit used in chemistry to express amounts of a chemical entity, it is defined as the amount of any substance that contains as many elementary constituents (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 grams of carbon-12 isotope this is the isotope of carbon with atomic mass 12. This corresponds to the Avogadro constant, which has a value of 6.023×1023 elementary constituents of the substance. It is one of the base units in the International System of Units, and has the unit symbol mol and corresponds with the dimension symbol N. In honor of the unit, some chemists celebrate October 23 as "Mole Day".
The unit mole is widely used in chemistry instead of units of mass or volume as it is a convenient way to express reactants or of products of chemical reactions.
2 H2 + O2 → 2 H2O
The above chemical reaction states that 2 moles of dihydrogen (H2) and 1 mole of dioxygen (O2) react to form 2 moles of water (H2O).
The mole may also be used to express the number of atoms, ions, or other elementary constituents in a given sample of any substance. The concentration of a solution is commonly expressed by its Molarity,
Molarity is defined as the “number of moles of the dissolved substance per litre of solution”.
The number of molecules in a mole is defined in way that the mass of one mole of a substance, expressed in grams, is exactly equal its mean molecular mass. For example, the mean molecular mass of natural water is approximately 18.00, so one mole of water is about 18.00 grams. Making use of this equation considerably simplifies many chemical and physical computations.
There is another term gram-molecule. This was formerly used for the same concept. The term gram-atom (abbreviated GAT) has been used for a related but distinct concept, namely a quantity of a substance that contains Avogadro's number of atoms or molecules, whether isolated or combined in molecules.
Example, 1 mole of MgBr2 is 1 gram-molecule of MgBr2 but 3 gram-atoms of MgBr2