Moles

Quite commonly we use the unit dozen to represent 12 articles irrespective of their nature. For example one dozen books mean 12 books whereas one dozen apples mean 12 apples. In a similar way chemists use the unit mole for counting atoms, molecules, ions, etc. A mole is a collection of 6.02X1023 particles.

Thus a mole represents 6.02X1023 particles.

The number 6.02X1023 is called as Avogadro's number.

Thus,

1 mole of pencil = 6.02X1023 number of pencils.

1 mole of hydrogen atoms = 6.02X1023 number of hydrogen atoms.

1 mole of hydrogen molecule =6.02X1023 number of hydrogen molecules

1 mole of sodium ion =6.02X1023 number of sodium ions

1 mole of electron = 6.02X1023 number of electrons

A curious question comes to our mind. Why scientist did chose the number 6.02X1023 for a mole? This seems to be a very odd choice. They could have chosen simple numbers like 103 106 0r 1020. However this choice is based on the fact that chemists preferred the definition in terms of a quantity that can be measured easily. We have learnt that atoms and molecules are so small that we cannot see them even with powerful microscope. Therefore, it's very difficult to count the actual number of atoms or molecules in a given sample of a substance. Thus weighing is easier than direct counting when the number of particles to be counted is very large. A mole was, therefore, originally defined in terms of mass. On this basis chemists selected a standard number, which is equal to the number of atoms present exactly in 12g of carbon (C12).

A mole is the amount of any substance containing as many particles as there are carbon atoms in exactly 12g of the carbon-12 isotope.

It may be noted that while using the term mole it is essential to specify the kind of particles involved. For example, One mole of oxygen atoms contain 6.02X1023 atoms of oxygen and weighs 16 g

One mole of oxygen molecules contain 6.02X1023 molecules of oxygen and weighs 32 g