How to count molecules

Molecules are immensely small and two molecules of water are really not much. Suppose you would like to describe a reaction forming one drop of water. A single drop of water contains 1,670,957,400,000,000,000,000 or 1.67x10²¹ 1.67 times ten to 21 water molecules. We know from the balanced chemical equation that it takes the same number of hydrogen molecules but only half that number of oxygen molecules for the reaction to occur. So we need 1.67x10²¹ 1.67 times ten to 21 hydrogen molecules and 8.35x10²⁰ 8.35 times ten to 20 oxygen molecules.

When counting molecules we are dealing with particles so tiny that they cannot be seen, even with a microscope, and with numbers so big that it is impossible to count. Therefore atoms and molecules are counted by mass, and in very large numbers called moles.

Just like eggs are often counted in dozens, so atoms and molecules are counted in moles. Don’t let the name confuse you, a mole is just a number.