When we mix two substances, we get one or more new substances. For example, when we mix hydrogen and oxygen and ignite the mixture, we get a new substance - water.

This can be represented in the form of a chemical equation,

2H2 (g) + O2 (g) → 2H2O (l)

In the above equation, 2 molecules (four atoms) of hydrogen react with 1 molecule (2 atoms) of oxygen and give two molecules of water. We always like to know how many atoms or molecules of a particular substance would react with atoms or molecules of another substance in a chemical reaction. The solution to this problem is to have a convenient unit. This chemical counting unit of atoms and molecules is called mole.


The word mole was, apparently introduced in about 1896 by Wilhelm Ostwald who derived the term from the Latin word ‘moles’ meaning a ‘heap’ or ‘pile’. The mole whose symbol is ‘mol’ is the SI (international system) base unit for measuring amount of substance.

A mole is the amount of substance that contains as many elementary entities (atoms, molecules, formula unit or other fundamental particles) as there are atoms in exactly 0.012 kg of carbon-12 isotope. In simple words, mole is the number of atoms in exactly 0.012 kg (12 grams) of C-12.

It is experimentally found that the number of atoms contained in exactly 12 g of C-12 is 6.022×1023. This number is called Avogadro’s number.

NA = 6.02×1023 mol–1

Molar Mass

Mass of one mole of a substance is called its molar mass. A substance may be an element or a compound. Mass of one mole atoms of oxygen means mass of 6.02×1023 atoms of oxygen. It is found that one mole atoms of oxygen weighs 16.0 g.

One mole molecules of oxygen means 6.02×1023 molecules of oxygen (O2). One mole molecules of oxygen will weigh 32.0 g. Thus,

Mass of one mole atoms of oxygen = 16 g mol–1

Mass of one mole molecules of oxygen = 32 g mol–1

1. Converting Moles to Grams

Example: How many grams are there in 3.5 mol of oxygen?

Molar mass of oxygen (O2) = 32 g mol–1

Therefore, number of grams of oxygen in 3.5 mol

= 3.5 mol of oxygen × 32.0 g mol–1

= 112.0 g of oxygen

2. Converting Grams to Molecules

Example: How many number of molecules are there in 27 g of water?

Mole concept provides a relationship between number of particles and their mass. Thus it is possible to calculate the number of particles in a given mass.

Number of mole of H2O = Mass of water (H2O) / Molar mass of H2O

= 27/18 = 1.5 mol

Since 1 mol of water contains 6.02×1023 molecules.

1.5 mol of water contains = 6.02 × 1023 molecules mol–1 × 1.5 mol

= 9.03 × 1023 molecules of water