# Mole Concept

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.

### 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