Published: 10 October, 2002

What's in a mole? It's a whole lot

What is a mole?

While riding around Ithaca, you may notice a car with personalized license plates reading 602E23. Why does the chemistry professor who owns this car have plates with that message? Because it represents the most important number in chemistry, Avogadro's number: 602,200,000,000,000,000,000,000.

This enormous number is more conveniently written 6.022 x 10²³, indicating that there are 23 digits to the right of the numeral 6. A mole is a measure of quantity. One mole of objects is Avogadro's number of those objects; one mole of eggs is 6.022 x 10²³ eggs. The idea of a mole is similar to that of a dozen, which is also a measure of quantity; one dozen eggs is 12 eggs. Eggs are sold by the dozen, because 12 eggs is a convenient quantity to have in your refrigerator at one time. One mole is also a convenient quantity for certain objects, but those objects must be much smaller than eggs.

We and the world around us are made of units, too small to be seen, called atoms, which in turn may be connected into molecules. The mole turns out to be a convenient quantity of atoms or molecules, because the objects around us that are big enough to see or touch contain approximately one mole of atoms or molecules. Chemists use the idea of the mole to figure out how many atoms or molecules are contained within a certain mass of material. For example, water is composed of molecules with the chemical formula H₂O, indicating that each molecule contains 2 atoms of hydrogen and one of oxygen. An 8 ounce glass of water contains approximately 13 moles of water molecules. For comparison, 13 moles of sand grains would cover all fifty United States to a depth of more than 40 feet. Here's a question for you. If you took a mole of pennies and stacked them into a single column, how many miles high would it reach?