What is a mole?

In chemistry the mole (mol) is a basic unit to describe a quantity of substance based on the number of particles (atoms or molecules) that it contains.

The number of particles in one mole of substance is 6.022x10^23, also known as the Avogadro constant.

Often when we set up chemical reactions we use relatively large masses of reactants, e.g. from few grams to kilograms. Even a gram contains billions and billions of particles, and it would be impractical to use such large numbers.

Because 1 mole = 6.022x10^23 particles, the mole acts as an easy way of referring to very large numbers of particles. The Avogadro constant acts as a reference large number.

Why is the Avogadro constant 6.022x10^23?

This is the number of atoms in 12 g of Carbon-12 (the relative atomic mass of the carbon atoms).