In the study of chemical changes apart from the qualitative aspects i.e. the name and nature of reactants and products, reaction conditions etc. the quantitative aspects are equally important for a chemist. The quantitative study involves the amount of various substances involved. This amount can be expressed in terms of either mass of substances or number of particles or volume of gaseous substances. In order to standardize the unit used to express the amount of substances, IUPAC introduced a fundamental unit called mole which is abbreviated as mol.
Mole is defined as
the amount of substance that contains as many entities as there are carbon atoms in exactly 12 gm of 12C isotope.
The number of atoms in 12 gm of 12C is called Avogadro’* number (NA or N0 or N). NA = 6.022 × 1023 entities
