What Is a Mole in Chemistry?

A mole is simply a unit of measurement. In fact, it's one of the seven base units in the International System of Units (SI). 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. So, scientists invented the mole to bridge the gap between very small and very large numbers.

Here is a look at what a mole is, why we use moles, and how to convert between moles and grams.

What Is a Mole?

Like all units, a mole has to be defined or else based on something reproducible. The present definition of the mole is defined, but it used to be based on the number of atoms in a sample of the isotope carbon-12.

Today, a mole is Avogadro's number of particles, which is exactly 6.02214076×10²³. For all practical purposes, the mass of one mole of a compound in grams is approximately equal to the mass of one molecule of the compound in daltons.

Originally, a mole was 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.

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Mole Definition in Chemistry

By Anne Marie Helmenstine, Ph.D.

Why We Use Moles

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. 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.

Sources

• Andreas, Birk; et al. (2011). "Determination of the Avogadro Constant by Counting the Atoms in a 28Si Crystal". Physical Review Letters. 106 (3): 30801. doi:10.1103/PhysRevLett.106.030801
• de Bièvre, Paul; Peiser, H. Steffen (1992). "'Atomic Weight' — The Name, Its History, Definition, and Units". Pure and Applied Chemistry. 64 (10): 1535–43. doi:10.1351/pac199264101535
• Himmelblau, David (1996). Basic Principles and Calculations in Chemical Engineering (6 ed.). ISBN 978-0-13-305798-0.
• International Bureau of Weights and Measures (2006). The International System of Units (SI) (8th ed.). ISBN 92-822-2213-6.
• Yunus A. Çengel; Boles, Michael A. (2002). Thermodynamics: An Engineering Approach (8th ed.). TN: McGraw Hill. ISBN 9780073398174.