Avogadro's Number Example Chemistry Problem

Avogadro
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by Anne Marie Helmenstine, Ph.D.
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Updated January 05, 2019

Avogadro's number is one of the most important constants used in chemistry. It is the number of particles in a single mole of a material, based on the number of atoms in exactly 12 grams of the isotope carbon-12. Although this number is a constant, it's experimentally determined, so we use an approximate value of 6.022 x 1023. So, you know how many atoms are in a mole. Here's how to use the information to determine the mass of a single atom.

Avogadro's Number Example Problem: Mass of a Single Atom

Question: Calculate the mass in grams of a single carbon (C) atom.

Solution

To calculate the mass of a single atom, first look up the atomic mass of carbon from the periodic table.
This number, 12.01, is the mass in grams of one mole of carbon. One mole of carbon is 6.022 x 1023 atoms of carbon (Avogadro's number). This relation is then used to 'convert' a carbon atom to grams by the ratio:

mass of 1 atom / 1 atom = mass of a mole of atoms / 6.022 x 1023 atoms

Plug in the atomic mass of carbon to solve for the mass of 1 atom:

mass of 1 atom = mass of a mole of atoms / 6.022 x 1023

mass of 1 C atom = 12.01 g / 6.022 x 1023 C atoms
mass of 1 C atom = 1.994 x 10-23 g

Answer

The mass of a single carbon atom is 1.994 x 10-23 g.

Applying the Formula to Solve for Other Atoms and Molecules

Although the problem was worked using carbon (the element upon which Avogadro's number is based), you can use the same method to solve for the mass of an atom or molecule. If you're finding the mass of an atom of a different element, just use that element's atomic mass.

If you want to use the relation to solve for the mass of a single molecule, there's an extra step. You need to add up the masses of all of the atoms in that one molecule and use them instead.

Let's say, for example, you want to know the mass of a single atom of water. From the formula (H2O), you know there are two hydrogen atoms and one oxygen atom. You use the periodic table to look up the mass of each atom (H is 1.01 and O is 16.00). Forming a water molecule gives you a mass of:

1.01 + 1.01 + 16.00 = 18.02 grams per mole of water

and you solve with:

mass of 1 molecule = mass of one mole of molecules / 6.022 x 1023

mass of 1 water molecule = 18.02 grams per mole / 6.022 x 1023 molecules per mole

mass of 1 water molecule = 2.992 x 10-23 grams