1894

The Mole

(Lorenzo Romano) Amedeo (Carlo) Avogadro (conte di Quaregna e Cerreto) (1776–1856), Stanislao Cannizzaro (1826–1910), Friedrich Wilhelm Ostwald (1853–1932)

In 1894, German chemist Friedrich Wilhelm Ostwald introduced the term mole into chemistry. Based on the work of Italian chemists Amedeo Avogadro and Stanislao Cannizzaro, it’s the amount (in grams) of any substance that matches its atomic or molecular weight. A mole of carbon (atomic weight 12) weighs 12 grams, and a mole of aspirin (molecular weight 180.16) weighs 180.16 grams. What makes it a useful concept is that a mole of any substance always has the same number of molecules in it, so if you have a chemical reaction in which Molecule A and Molecule B react together in a one-to-one ratio, you can simply add one mole of each. It can be a quarter of a mole each, or a thousandth of a mole each (one millimole), but as long as you work in molar ratios, everything will come out even.

For the many non-chemists who have encountered the idea of a mole and been confused by it, here’s another way to think about it: It’s a big dozen. A dozen eggs, a dozen bowling balls, a dozen elephants—they weigh different amounts, but there are twelve of each of them. A mole of oxygen, a mole of plutonium chloride (stand back), a mole of insulin—they weigh different amounts, but there’s an Avogadro’s number of molecules in each of them.

So, how many molecules does a mole of anything contain? 6.022141 × 10²³, to be exact, which is known as Avogadro’s number, to honor his work with molecular weights and ratios. That’s a huge number. One mole of carbon isn’t much—the graphite from a dozen pencils, more or less—but there are over six hundred thousand million million million carbon atoms in there. Even reagent solutions in the lab are labeled in terms of how many moles of compound they have in them per liter. Thinking in terms of moles is second nature to chemists, allowing them to keep track of ratios and equivalents in chemical reactions without worrying about molecular weights.

One mole of nitrogen (in the balloon), with a mole each of aluminum, iron, copper, sodium (as sodium chloride), and mercury in front of it.