Chemistry - the Complete Course Series, teaches students quantitative reasoning to solve chemistry problems. The 30 volume series covers all aspects of Chemistry from density to the mole, to molarity, stoichiometry and equilibrium. It demonstrates how to ask meaningful questions and conduct careful investigations to build knowledge of chemical and physical concepts and develop the skills to solve mathematical and scientific problems. Chemists invented the concept of the mole because they needed a way to "count" atoms and molecules. We need to be able to "count out" atoms and molecules in order to run chemical reactions without having anything left over. This volume teaches the mole is defined as a quantity of a substance whose mass is equal to it's atomic or molecular weight expressed in grams instead of amu's. a mole of any substance contains the same number of "particles" as a mole of any other substance. A mole is a quantity that is very analogous to a dozen.