What Is a Mole and Why Are Moles Used?

Understanding the Mole Unit of Measurement

Amedeo Avogadro
Avogadro's Number is named after Italian scientist Amedeo Avogadro. Bettmann / Contributor / Getty Images

What Is a Mole and Why Are Moles Used?

A mole is simply a unit of measurement. Units are invented when existing units are inadequate. Chemical reactions often take place at levels where using grams wouldn't make sense, yet using absolute numbers of atoms/molecules/ions would be confusing, too.

Like all units, a mole has to be based on something reproducible. A mole is the quantity of anything that has the same number of particles found in 12.000 grams of carbon-12.

That number of particles is Avogadro's Number, which is roughly 6.02x1023. A mole of carbon atoms is 6.02x1023 carbon atoms. A mole of chemistry teachers is 6.02x1023 chemistry teachers. It's a lot easier to write the word 'mole' than to write '6.02x1023' anytime you want to refer to a large number of things! Basically, that's why this particular unit was invented.

Why don't we simply stick with units like grams (and nanograms and kilograms, etc.)? The answer is that moles give us a consistent method to convert between atoms/molecules and grams. It's simply a convenient unit to use when performing calculations. Okay... you may not find it too convenient when you are first learning how to use it, but once you become familiar with it, a mole will be as normal a unit as, say, a dozen or a byte.

Converting Moles To Grams

One of the most common chemistry calculations is converting moles of a substance into grams.

When you balance equations, you'll use the mole ratio between reactants and reagents. To do this conversion, all you need is a periodic table or another list of atomic masses.

Example: How many grams of carbon dioxide is 0.2 moles of CO2?

Look up the atomic masses of carbon and oxygen. This is the number of grams per one mole of atoms.

Carbon (C) has 12.01 grams per mole.
Oxygen (O) has 16.00 grams per mole.

One molecule of carbon dioxide contains 1 carbon atom and 2 oxygen atoms, so:

number of grams per mole CO2 = 12.01 + [2 x 16.00]
number of grams per mole CO2 = 12.01 + 32.00
number of grams per mole CO2 = 44.01 gram/mole

Simply multiply this number of grams per mole times the number of moles you have in order to get the final answer:

grams in 0.2 moles of CO2 = 0.2 moles x 44.01 grams/mole
grams in 0.2 moles of CO2 = 8.80 grams

It's good practice to make certain units cancel out to give you the one you need. In this case, the moles canceled out of the calculation, leaving you with grams.

You can also convert grams to moles.

Additional link: Learn About Mole Day