By Rosann Kozlowski – Updated Aug 30, 2022
To translate a quantity of moles into the exact number of molecules, you’ll need the sample’s mass, the molar mass derived from the periodic table, and Avogadro’s constant (6.022 × 10²³ molecules / mol). A handy online calculator can handle the final conversion, but the key steps remain the same.
Locate each element’s atomic mass on a periodic table. For a single‑element sample, the atomic mass equals the molar mass. For a compound, add the masses of all constituent atoms. Example: H₂O has 2 × 1.01 amu + 16.00 amu = 18.02 amu, which is 18.02 g / mol.
Example problem: 60.50 g of CaCl₂
Use the molar mass to find the number of moles:
60.50 g × (1 mol / 110.98 g) = 0.545 mol of CaCl₂ (rounded to three significant figures).
Multiply the moles by Avogadro’s constant:
0.545 mol × (6.022 × 10²³ molecules / mol) = 3.28 × 10²³ molecules of CaCl₂.
Steps 2 and 3 can be combined into a single conversion factor:
60.50 g × (1 mol / 110.98 g) × (6.022 × 10²³ molecules / 1 mol) = 3.28 × 10²³ molecules.
Online tools like the Omni Calculator simplify the process, but you still need to compute the molar mass yourself.
The mole is the SI unit that represents 6.022 × 10²³ entities, whether atoms, ions, molecules, or even macroscopic objects. This number was refined to 6.02214076 × 10²³ by modern experiments and is the same for all substances.
An atom is the smallest unit of an element that retains its chemical identity, consisting of a nucleus of protons and neutrons surrounded by electrons. Molecules are groups of atoms held together by chemical bonds—ionic, covalent, or metallic—forming a compound with a fixed ratio of elements.
The concept originates from Amedeo Avogadro, who proposed that equal volumes of gases at identical conditions contain the same number of particles. Though the constant was first measured by Josef Loschmidt and later formalized by Jean Perrin, it honors Avogadro’s insight. The current accepted value, 6.02214076 × 10²³, is maintained by the National Institute of Standards and Technology.
