Large number of physical and chemical experiments depends upon measurements and calculation of various parameters. Any measurement always has two parts – numerical part and a unit. For e.g. let’s say the mass of a ball is 2kg, in this 2 is numerical part and kg is unit. 1kg is a well defined unit, similarly for most of other measurements we have well defined units, like Kelvin is for temperature, meter is for length etc. But what about counting, does it have any unit. Many may answer it as no, that there’s no unit of counting. It’s true that there is no S.I unit of counting but nevertheless, we have units of counting. One such kind of unit is a dozen. It is equivalent to 12. So 1 dozen apples means 12 apples. Similar kind of unit for counting was required when the sub-atomic particles were involved as the scientists started getting very large numbers frequently. At that time, concept of mole was introduced in the study. Like dozen, it is also a unit for counting. It is used to count very large numbers. In this section we shall learn about mole concept.

**1 mole = 6.022 x 10 ^{23}**

This number is known as Avogadro’s constant and is denoted by N_{A}.

**Definition**

A mole is defined as the amount of any chemical substance that contains as many elementary entities, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of pure carbon-12 (^{12}C).

**Facts about Mole concept**

- Mole concept is one of the most convenient ways of expressing amount of reactants and product in reaction.
- The word “mole” was introduced around 1896 by Wilhelm Ostwald who derived the term from the Latin word
*moles*meaning a ‘heap’ or ‘pile. - The mass of one mole of substance is equal to the gram formula mass of that substance.

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