ConverterHub

Moles Calculator

Find moles from mass and molar mass.

How it is calculated

The amount of substance in moles is the mass divided by the molar mass: n = m / M. Here m is the mass of the sample in grams and M is the molar mass in grams per mole (g/mol). The tool does not look up compounds, so you add the molar mass yourself: sum the atomic masses on the periodic table, each multiplied by the number of atoms of that element in the formula. For water, H₂O, that is 2 × 1.008 + 15.999 ≈ 18.015 g/mol. The molar mass must be above zero, otherwise no result is shown. The answer is rounded to two decimals and shown in mol. To go the other way, multiply moles by M to get grams, or by 6.022 × 10²³ to count particles.

Worked example

How many moles are in 36 g of water? The molar mass of H₂O is 18.015 g/mol. Divide the mass by the molar mass: 36 / 18.015 = 1.998. Rounded to two decimals the tool shows 2.00 mol. That is almost exactly two moles, which makes sense because 36 g is about twice the 18 g in one mole of water. Using that result in a reaction, the sample contains about 2 × 6.022 × 10²³ water molecules.

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Common mistakes

  • Using the atomic mass of one atom instead of the molar mass of the whole formula, for example 16 for H₂O instead of 18.
  • Entering the mass in kilograms or milligrams while the molar mass is in g/mol. Convert the mass to grams first.
  • Forgetting to multiply by subscripts, such as using 12 for CO₂ instead of 12 + 2 × 16 = 44 g/mol.
  • Ignoring hydrates, where the water in the crystal counts toward the molar mass (CuSO₄·5H₂O is far heavier than CuSO₄).

About this calculator

This calculator applies the basic stoichiometry relationship n = mass ÷ molar mass, where molar mass (in g/mol) comes from summing the atomic weights of every atom in the compound's formula.

It's used by chemistry students converting between grams and moles for lab reports, lab technicians preparing solutions of a specific molarity, and pharmacy or manufacturing staff calculating reagent quantities for a recipe or synthesis.

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