Think of a word used to describe a large number, how about a billion. Well, in number form, a billion is 1,000,000,000. Otherwise, a very large number, with nine zeros. That�s why when you write about a billion things, you write the word �billion� instead of the actual number, because the actual number is huge.

Now what if you wanted a word to describe how many water molecules are in a glass of water. As you know, water molecules (H₂O) are very very small, so there has to be a lot of these very very small molecules. Well, there are so many water molecules in that glass of water, that a billion doesn�t even come close to describing how many there are in a glass of water. That�s why a scientist decided to use a word of his own to describe a really large number that he used to measure the number of molecules. This scientist is named Avogadro, and this is Avogadro�s number :

Now why this particular number to use as a mole? Well, technically, it is the number equal to the number of carbon atoms in exactly 12 grams of pure 12C(Carbon with exactly 6 protons and 6 neutrons). Now this number is very big! Just imagine a mole as a name for a number, like a dozen means twelve. Except, that a mole is a really large number. One mole of marbles could cover the entire earth over 50 miles deep! This number is the basis for most chemical calculations simply because it shows how many molecules or atoms there are!

This can help out when someone is trying to combine enough carbon tetrafluoride (CH₄) with enough Oxygen (O₂) so the reaction will carry out completely with no CH₄ or O₂ left over. Here�s the chemical equation: