Moles: the Number of Water molecules
According to the atomic theory, atoms are the units of chemical reactions. The formula H2O indicates that each molecule of this substance contains one oxygen and two hydrogen atoms. Therefore, if we ask how much hydrogen is required to make a given quantity of water, the answer is two hydrogen atoms for each oxygen atom or two hydrogen atoms per molecule. In other words, how much substance we have depends in a very important way on how many atoms or molecules are present.
So far, we've dealt with mass ratios. Is there a way to change masses of atoms into numbers of atoms, so it is easy to see how much of one element will react with another, just by looking at the number of atoms that are needed?
"How much?" in the above sense of the quantity of atoms or molecules present is not the same thing as how much in terms of volume or mass. It takes 15.994 g of oxygen to react with 2.016 g of hydrogen. In the atomic sense, however, just 2.016 g of hydrogen contains twice as many hydrogen atoms as there are oxygen atoms in 15.994 g of oxygen, and twice the amount of hydrogen as oxygen.
Luckily, the International System of Measurements (IUPAC) has a measure of amount that reflects the number of atoms present, and it is called the mole.