How to Calculate Moles from Mass

Finds the amount of substance as mass ÷ molar mass. One mole is 6.02214076 × 10²³ particles, the Avogadro constant.

Explanation

The mole is a way of counting particles. Atoms and molecules are far too small to count one by one, so chemists bundle 6.02214076 × 10²³ of them together and call that one mole. The number is the Avogadro constant.

n = \dfrac{m}{M}

Here $n$ is the amount in moles, $m$ the mass in grams and $M$ the molar mass in grams per mole. The molar mass is simply the atomic or molecular mass expressed in grams.

Example

Find the amount in 18 g of water. Water, $\mathrm{H_2O}$ , has a molar mass of $1 \times 2 + 16 = 18$ g/mol.

n = \dfrac{18}{18} = 1\ \text{mol}

That is $6.02 \times 10^{23}$ molecules. A tablespoon of water contains six hundred thousand billion billion molecules.

Molar masses worth knowing

Water $\mathrm{H_2O}$ — 18 g/mol
Carbon dioxide $\mathrm{CO_2}$ — 44 g/mol
Oxygen $\mathrm{O_2}$ — 32 g/mol
Sodium chloride $\mathrm{NaCl}$ — 58.5 g/mol
Glucose $\mathrm{C_6H_{12}O_6}$ — 180 g/mol

Watch out

A molar mass is just the sum of the atomic masses in the formula. For $\mathrm{H_2O}$ that is two hydrogens (1 × 2) plus one oxygen (16), giving 18. Do not forget to multiply by the subscripts.

Since the 2019 revision of the SI, the Avogadro constant is a defined value. It is no longer measured: a mole simply *is* that many particles, by definition.