The mole is a unit of measurement for the amount of substance or chemical amount. It is one of the base units in the International System of Units, and has the unit symbol mol.

The mole is defined as the amount of substance that contains as many elementary entities (atoms, molecules, ions, electrons) as there are atoms in 12g of the isotope carbon - 12. Thus, by definition, one mole of pure 12C has a mass of exactly 12g. The experimentally determined value of a mole is

A mole of any pure substance has a mass in grams approximately equal to that substance's molecular or atomic mass; e.g., 1 mol of calcium - 40 has an approximative mass of 40g, because the Ca - 40 isotope has a mass of 39.9625906 amu on the C - 12 scale. In other words, the numerical value of a substance's molecular or atomic mass in atomic mass units is the same as that of its molar mass - the mass of one mole of that substance in grams.

The most common method of determining the amount, expressed in moles, of pure substance the value of whose molar mass is known, is to measure its mass in grams and then to divide by its molar mass Molar masses may be easily calculated from tabulated values of atomic weights and the molar mass constant (which has a convenient defined value of 1 g/mol). Other methods include the use of the molar volume or the measurement of electric charge.

The mol appears in the kinetic theory of gases via the ideal gas equationwhere n is the number of mols and is the molar gas constant, and it is the same for all gases. From this equation we find that the volume of 1 mol of gas at 0 degrees Celsius and atmospheric pressureis