Amount of substance is a standards-defined
quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The
International System of Units (SI) defines the amount of substance to be
proportional to the
number of elementary entities present. The SI unit for amount of substance is the
mole. It has the unit symbol mol. The proportionality constant is the inverse of the
The mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 12
g of the isotope
 This number is called Avogadro's number
 and has the value 140857(74)×1023. 6.022
 It is the numerical value of the Avogadro constant which has the unit mol-1, and relates the
molar mass of an amount of substance to its mass. Therefore, the amount of substance of a sample is calculated as the sample mass divided by the
molar mass of the substance.
Amount of substance appears in thermodynamic relations such as the
ideal gas law, and in stoichiometric relations between reacting molecules as in the
law of multiple proportions.
Another unit of amount of substance in use in
chemical engineering in the United States is the
pound-mole, having the symbol lb-mol.
 One pound-mole is 37 mol. 453.592