What are moles in chemistry?

A mole is a unit of the quantity of matter. It is a unit that expresses the amount of matter. One mole of a substance is equal to the amount of that substance that has 6.022e23 entities in it. By entities we mean the basic particles of a substance; elements have atoms and compounds have molecules. Therefore one mole of an element is 6.022e23 atoms of that element and one mole of a compound is 6.022e23 molecules of that compound. 6.022e23 is Avogadro's number.

The mole concept in chemistry states that the amount of a substance equal to one mole of it will have a mass equal to its relative molecular or atomic mass (according to whether it is an element or a compound) expressed in grams instead of Atomic Mass Units. Therefore one mole of Hydrogen atoms (H) will have a mass of 1 grams and 1 mole of Oxygen molecules (O2) will have a mass of 16 grams approximately.

Example of a question relating to mole concept in chemistry:

Find the mass of water that will form from 3 moles of Hydrogen reacting with sufficient Oxygen according to the chemical equation:

H2 + O2 -> H2O

Solution:

First balancing the chemical equation, we get:

2H2 + O2 -> 2H2O

Now since 2 molecules of Hydrogen (H2) form 2 molecules of water (H2O) therefore 3 moles of Hydrogen forms 3 moles of water. Mass of 3 moles of water is:
Molecular mass of water is: 2(1) + 16 = 18
Therefore 1 mole of water molecules has a mass of 18 grams.
Therefore 3 moles of water molecules have a mass of 3 * 18 = 54 grams.

Therefore 3 moles of hydrogen molecules on reaction with oxygen form 54 grams of water molecules.