One of the central concepts of chemical stoichiometry is the mole concept. It is defined as the amount of material that contains as many elementary units as there are atoms in 12 gram of the C-12 isotope of carbon. This number is actually 6.0222 × 10˄23, also known as Avogadro number, in honour of the great scientist who first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules regardless of the nature of the gas. The mole concept has simplified the way of talking about number of entities without worrying about its mass. An atom or molecule is an extremely small entity. In a very small physical space, one can align between 2-10 million atoms! Dealing with such a huge number is not so convenient always; mole, being a very large unit, has simplified such calculations and helped stoichiometric calculations even simpler. Another great advantage of mole concept is that, if we take weights of different elements in the same ratio of their atomic masses, then we have identical number of atoms. For example, suppose we take 12.0 gram of carbon (C-12). We would have 6.0222 × 10˄23 numbers of atoms in our hand. Now suppose further that we take 16.0 grams of oxygen, or 2.008 grams of hydrogen, we would then have the same number of oxygen atoms and twice its number of hydrogen atoms.

Moreover, the mole concept has helped maintain precise control over relative number of atoms going into a chemical reaction at whatever scale, and thence on the nature of the product formed. Thus, while brominating a substance which has multiple bromination sites, a substrate to bromine mole ratio of 1:1, would yield one product, while a 1:3 mole ratio would almost certainly yield a different product.