By Michael Judge | Updated Aug 30, 2022

In chemistry, a mole is the unit that links the mass of a substance to the number of molecules or atoms it contains. One mole equals 6.022 × 10²³ entities—Avogadro’s number—making it an essential tool for quantitative analysis.

Step 1 – Identify the Molecular Formula

Start with the exact molecular formula of the compound. The formula tells you how many atoms of each element are present in one molecule. For example, water is H₂O, meaning each molecule contains two hydrogen atoms and one oxygen atom.

Step 2 – Retrieve Atomic Weights

Consult a periodic table to find the atomic weight (also called atomic mass) of each element in the formula. Typical values are 1.008 g/mol for hydrogen and 16.00 g/mol for oxygen.

Step 3 – Compute the Molar Mass

Multiply each element’s atomic weight by the number of atoms of that element, then sum the results:

(2 × 1.008) + (1 × 16.00) = 18.016 g/mol

This total, expressed in grams per mole, is the molar mass of the compound.

Step 4 – Convert Mass to Moles

Divide the sample’s mass (in grams) by the molar mass to obtain the number of moles:

25 g H₂O ÷ 18.016 g/mol ≈ 1.39 mol

Tools You’ll Need

• Periodic table (print or digital)
• Scientific calculator

Follow these steps to calculate moles for any chemical, whether you’re working in a lab or studying chemistry coursework.

Mastering mole calculations unlocks the ability to perform accurate stoichiometric calculations, design experiments, and interpret analytical data.