How to Calculate the Number of Molecules

By Oxana Fox; Updated June 09, 2017
You need to know the mass to calculate how many molecules of the salt is here.

In chemistry, a mole is a quantity unit that stands for the amount of a given substance. Since one mole of any chemical compound always contains 6.022 x 10^23 molecules, you can calculate the number of molecules of any substance if you know its mass and its chemical formula. The number 6.022 x 10^23 is called as the Avogadro constant.

1. Get the Chemical Formula

Get the chemical formula of the compound. For example, if the compound is sodium sulfate, Na2SO4, each molecule contains two atoms of sodium (Na), one atom of sulfur (S) and four atoms of oxygen (O).

2. Get the Atomic Weights of Each Element

Find the element symbols in the Periodic Table of the Elements, and write down the atomic weights of each element. In our example, the atomic weight of sodium (Na) is 23; sulfur (S) is 32; and oxygen (O) is 16.

3. Calculate the Atomic Weight of the Compound

Multiply the atomic weight of each element by the number of atoms of the element in the molecule, and add these to calculate the compound molar mass. In the example, the molar mass of Na2SO4 is (23 x 2) + (32 x 1) + (16 x 4) = 142 grams per mole.

4. Calculate the Number of Moles

Divide the known mass of the compound by its molar mass to calculate the number of moles. For example, suppose the mass of your sample of Na2SO4 is 20 g. The number of moles equals 20 grams /142 grams/mole = 0.141 moles.

5. Multiply Moles by the Avogadro Constant

Multiply the number of moles by the Avogadro constant, 6.022 x 10^23, to calculate the number of molecules in your sample. In the example, the number of molecules of Na2SO4 is 0.141 x 6.022 x 10^23, or 8.491 x 10^22 molecules of Na2SO4.

About the Author

Oxana Fox is a freelance writer specializing in medicine and treatment, computer software and hardware, digital photography and financial services. She graduated from Moscow Medical College in 1988 with formal training in pediatrics.