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Mole as a Unit of Measurement of Amount of Substance

In a chemical investigations, we are often interested in knowing the number of atoms and molecules we are dealing with. For example, let us consider the following reaction:

N2 + 3H2 2NH3

In this reaction, one molecule of nitrogen reacts with three molecules of hydrogen. So, it would be desirable to take the molecules of nitrogen and hydrogen in the ratio 1 : 3, so that the reactants are completely consumed during the reaction. But we know atoms and molecules are so small in size that it is not possible to count them individually. Moreover, the smallest sample of the substance that we may take contains a very large number of atoms or molecules. In order to

overcome these difficulties, the concept of mole was introduced. According to this concept, number of particles of the substance is related to mass of the substance. For the sake of counting elementary particles number of atoms in 12 g of carbon-12 isotope is taken as standard. This number has been experimentally found to be 6.023 x 1023 and is called one mole. This number is also known as Avogadro’s number or Avogadro constant (N A).

The mole* may be defined as “the amount of the substance that contains as many specified elemental} particles (atoms, molecules, ions or electrons etc.,) as the number of atoms in 12 g of carbon-12 isotope”.

Mole is thus a unit for counting atoms, molecules, ions. etc. It stands for 6.023 x 1023 particles irrespective of their nature. The tern mole should not be confused with the physical quantity, amount of the substance because amount of the substance is not a number.

SUMMARY of various relationships of mole has been illustrated as follows in Tabular form.

The word mole was introduced by Wilhelm Ostwald who derived the term from the Latin word moles meaning a heap or pile. A substance may be considered as heap of atoms or molecules. In 1967, mole was accepted as a unit which provides a simple way of reporting a large number (6.023 x 1023), i.e., a massive heap of atoms or molecules in a sample.