Moles Calculator — grams to moles (n = mass ÷ molar mass)

How it works

The mole is chemistry's unit for amount of substance — one mole is 6.022×10²³ particles (Avogadro's number). To connect a measurable mass to a number of moles, you use the substance's molar mass, the mass of one mole in grams per mole (OpenStax Chemistry):

Moles = mass ÷ molar mass

A substance's molar mass in g/mol is numerically equal to its formula (molecular) mass in atomic mass units, found by adding up the atomic masses of every atom in its formula. Water (H₂O) is 2×1.008 + 16.00 ≈ 18.015 g/mol; sodium chloride (NaCl) is 22.99 + 35.45 ≈ 58.44 g/mol. Once you have the molar mass, dividing your sample's mass by it gives the number of moles — and multiplying moles by molar mass converts back to mass.

Working in moles matters because chemical reactions combine substances in whole-number mole ratios, not mass ratios. Converting grams to moles is usually the first step in any stoichiometry problem.

Worked example

For 90 g of glucose, whose molar mass is 180.16 g/mol:

Moles = 90 ÷ 180.16 ≈ 0.4996 mol (about half a mole)

And 18.015 g of water divided by its 18.015 g/mol molar mass is exactly 1 mole.

Frequently asked questions

How do you convert grams to moles?

Divide the mass in grams by the molar mass in g/mol. For 90 g of glucose (180.16 g/mol), that is 90 ÷ 180.16 ≈ 0.50 mol.

What is molar mass?

Molar mass is the mass of one mole of a substance, in grams per mole. It equals the sum of the atomic masses of all atoms in the chemical formula, read off the periodic table.

How do I find a compound's molar mass?

Add the atomic masses of every atom in its formula. For water (H₂O): 2 × 1.008 (hydrogen) + 16.00 (oxygen) ≈ 18.015 g/mol.

How do I convert moles back to grams?

Multiply the number of moles by the molar mass: mass = moles × molar mass. Half a mole of water is 0.5 × 18.015 ≈ 9.0 g.

What is a mole?

A mole is an amount of substance containing exactly 6.022×10²³ particles (Avogadro's number) — atoms, molecules or ions. It lets chemists count particles by weighing.

Why work in moles instead of grams?

Because reactions combine substances in fixed whole-number mole ratios, not mass ratios. Converting to moles is the first step in stoichiometry, letting you relate amounts of reactants and products.