Mole concept

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

The term mole came 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 usually introduced at the beginning of an upper secondary school Chemistry syllabus.

What is it?

Just as a dozen is 12 and a hundred is 100, the mole, which is also known as the Avogadro number is simply a number that is equal to 6.02214076 x 1023. It is used in the same way you would for a dozen, where 2 dozens is 2 x 12 = 24, and so 5 moles is:

5 x 6.02214076 x 1023 = 3.01107 x 1024

The concept of ‘dozen’ is useful in describing small amounts of things 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 dm3, 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 1023 and we can express the relationship between number of moles and number as:

number = number of moles x 6.02214076 x 1023                (1)

Because the quantity 6.02214076 x 1023 is a constant, we replace it with a symbol, NA:

number = nNA

where n is the number of moles with the unit mol and Nis the Avogadro constant, which has the unit mol-1.

next article: how was the avogadro constant determined?
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