<P> 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 Avogadro constant . </P> <P> 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 carbon - 12 . This number is called Avogadro's number and has the value 7023602214085700000 ♠ 6.022 140 857 (74) × 10 . It is the numerical value of the Avogadro constant which has the unit mol, 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 . </P>

The is the si unit that expresses the amount of substance