In chemistry the mole is a fundamental unit in the Système International d'Unités, the SI system, and it is used to measure the amount of substance. This quantity is sometimes referred to as thechemical amount.In Latinmolemeans a "massive heap" of material. It is convenient to think of a chemical mole as such.Visualizing a mole as a pile of particles, however, is just one way to understand this concept. A sample of a substance hasa mass, volume (generally used with gases), and number of particles that is proportional to the chemical amount (measured in moles) of the sample. For example, one mole of oxygen gas (O2) occupies a volume of 22.4 L at standard temperature and pressure (STP; 0°C and 1 atm), has a mass of 31.998 grams, and contains about 6.022 × 1023molecules ofoxygen. Measuring one of these quantitiesallows the calculation of the others and this is frequently done in stoichiometry.Themoleis to theamount of substance(or chemical amount) as thegramis tomass.Like other units of the SI system, prefixes can be used with the mole, so it ispermissible to refer to 0.001 mol as 1 mmol just as 0.001 g is equivalent to 1 mg.Formal DefinitionAccording to the National Institute of Standards and Technology (NIST), the Fourteenth Conférence Générale des Poids et Mesures established the definition of the mole in 1971.The mole is the amount of a substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12; its symbol is "mol." When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.