The mole concept is a way to quantify atoms and molecules.

The term ** mole** comes from the word

*molecule*and was coined by the German chemist Friedrich Ostwald in 1894. It is one of the most fundamental concepts in physical chemistry and is usually introduced at the beginning of an upper secondary school Chemistry syllabus.

##### What is it?

Just as a dozen is 12, the mole is a unit of measurement defined by the **Avogadro number**, which is simply a number equal to 6.02214076 x 10^{23}. It is used in the same way you would use a dozen. For example, 2 dozens is 2 x 12 = 24, so 5 moles is:

5 x 6.02214076 x 10^{23} = 3.01107 x 10^{24}

The concept of ‘dozen’ is useful in describing small quantities of items like eggs. The mole, on the other hand, is handy when we want to account for trillions of a trillion (septillion) of atoms or molecules within a small volume of space. Instead of saying that there are 3,011,000,000,000,000,000,000,000 oxygen molecules in 120 dm^{3}, we say that there are 5 moles of oxygen molecules within the same space.

Since ‘number of dozens’ is a multiple of 12, the relationship between number of dozens and **number** is:

**number** = number of dozens x 12

With reference to the above equation, ‘number of dozens’ has the unit of *dozen*, while 12 is a constant with the unit *dozen ^{-1}*. Hence, when we multiply the two terms, we get

**number**, which is unit-less. Similarly, ‘number of moles‘ is a multiple of 6.02214076 x 10

^{23}and we can express the relationship between number of moles and number as:

**number** = number of moles x 6.02214076 x 10^{23 }(1)

Because the quantity 6.02214076 x 10^{23} is a constant, we replace it with a symbol, *N _{A}*:

**number** =* n* x *N _{A}*

where *n* is the number of moles with the unit *mol* and *N _{A }*is the

**Avogadro constant**, which has the unit

*mol*.

^{-1}