Defining the mole

The world mole stands for a number — approximately 6.022 × 10²³. It’s commonly called Avogadro’s number, named for Amedeo Avogadro, the scientist who laid the groundwork for the mole principle.

Now a mole — 6.022 × 10²³ — is a really big number. When written in longhand notation, it’s 602,200,000,000,000,000,000,000

And that is why I like scientific notation. (If you had a mole of marshmallows, it’d cover the United States to a depth of about 600 miles. A mole of rice grains would cover the land area of the world to a depth of about 75 meters.)

Avogadro’s number stands for a certain number of things. Normally, those things are atoms and molecules. So the mole relates to the microscopic world of atoms and molecules. But how does it relate to the macroscopic world where you work? The answer is that a mole (abbreviated as mol) is equal to the following:

✓ For carbon: A mole is the number of atoms in exactly

12 grams of C-12, a particular isotope of carbon. So if you have exactly 12 grams of 12C, you have 6.022 × 10²³ carbon atoms, which is also a mole of 12C atoms.

✓ For any other element: A mole is the atomic weight using grams instead of atomic mass units.

For a compound: For a compound, a mole is the formula (or molecular) weight in grams instead of atomic mass units.