I’m not talking about the ground burrowing lawn-destroying rodents, nor am I talking about the Mexican chocolate sauce. Neither am I talking about spies, or ugly brown things on people’s skin. I’m referring to the unit of measurement for chemical substances.


A mole refers to 6.02 x 1023 of any given substance. For example, a mole of iron contains 6.02 x 1023 atoms of iron.


Moles are important because atoms have different masses. Hydrogen (atomic number 1) has a much smaller mass than oxygen (atomic number 8). Imagine that you are trying to create water out of oxygen and water given the following chemical reaction:


2H2 + O2 > 2H2O


Intuitively, one might expect to be able to use mass for this reaction. The problem is that 2 grams of H2 have 6.02 x 1023 molecules and one gram of O2 has 1.88 x 1022 molecules. This is because, as explained earlier, oxygen is much heavier than hydrogen. Because the equation demands 2 H2 molecules per oxygen molecule, there will be a significant excess of hydrogen left after the chemical reaction. There will be 5.64 x 1023 extra molecules.


To solve this problem, scientists measure the quantities used in reactions in terms of the number of molecules. Historically, one mole was defined as the amount of atoms in 1 gram of hydrogen. Since then, the definition has changed. Now one mole is defined as 12 grams of pure carbon-12 (6.02 x 1023).


Let’s redo the previous chemical reaction, using moles instead of grams. 2 moles of H2 has a mass slightly north of 4 grams. One mole of oxygen has a molar mass of 32 grams. By combining 4 grams (or 2 moles) of H2 and 32 grams (or one mole) of O2 the result is 36 grams of water with no excess.


And now you understand moles.