What Is a Mole in Chemistry? Definition and Examples
A mole is the amount of a substance that contains exactly 6.02214076 × 10²³ particles (atoms, molecules or ions). This number is called Avogadro’s number. One mole of any substance has a mass in grams equal to its relative formula mass, which lets chemists count particles by weighing them.
What is Avogadro's number?
Avogadro's number (the Avogadro constant) is 6.02214076 × 10²³ per mole. It is named after the Italian scientist Amedeo Avogadro, and since 2019 the value is fixed exactly in the SI system.
To get a sense of its size, a mole of water is only 18 g, about a tablespoon, but it contains 6.02 × 10²³ molecules.
The mole definition in plain terms
There are two ways to say it, and both are useful:
- Count: 1 mol = 6.02 × 10²³ particles.
- Mass: 1 mol of a substance has a mass in grams equal to its relative atomic or formula mass, called the molar mass.
So 1 mol of carbon atoms has a mass of 12.0 g, 1 mol of water has a mass of 18.0 g and 1 mol of sodium chloride has a mass of 58.5 g. Each of these samples contains 6.02 × 10²³ particles.
The moles formula
The moles formula links three quantities:
n = m ÷ M
| Symbol | Meaning | Unit |
|---|---|---|
| n | Amount of substance | mol |
| m | Mass | g |
| M | Molar mass | g/mol |
Rearranged, m = n × M and M = m ÷ n. To count the particles, multiply by the Avogadro constant: N = n × 6.02 × 10²³.
To work out M, add the relative atomic masses from the periodic table. For water, H₂O: 2 × 1.0 + 16.0 = 18.0 g/mol. If you need a step-by-step method, read how to find molar mass.
How to convert grams to moles
To convert grams to moles, follow these steps:
- Write the chemical formula.
- Work out the molar mass M by adding the atomic masses.
- Divide the mass by the molar mass: n = m ÷ M.
- Include the unit (mol).
Example 1: grams to moles
How many moles are in 11.0 g of carbon dioxide?
- M(CO₂) = 12.0 + 2 × 16.0 = 44.0 g/mol.
- n = 11.0 ÷ 44.0 = 0.25 mol.
Example 2: moles to particles
How many molecules are in 0.25 mol of CO₂?
- N = 0.25 × 6.02 × 10²³ = 1.5 × 10²³ molecules.
Example 3: mass from moles
What is the mass of 2.0 mol of oxygen gas, O₂?
- M(O₂) = 2 × 16.0 = 32.0 g/mol.
- m = n × M = 2.0 × 32.0 = 64 g.
Example 4: ionic compounds
How many moles are in 5.85 g of sodium chloride? M(NaCl) = 23.0 + 35.5 = 58.5 g/mol, so n = 5.85 ÷ 58.5 = 0.100 mol. That is 0.100 × 6.02 × 10²³ = 6.02 × 10²² formula units.
Moles and gases
At room temperature and pressure (20 °C and 1 atm), 1 mole of any gas occupies about 24 dm³. At standard temperature and pressure (0 °C and 1 atm), the molar volume is about 22.4 dm³. This is because gas volume depends on the number of particles, not on their type. See what is room temperature for why the reference temperature matters.
Why do chemists use moles in equations?
In a balanced equation, the coefficients are mole ratios. In 2H₂ + O₂ → 2H₂O, 2 mol of hydrogen reacts with 1 mol of oxygen to make 2 mol of water. If you know the moles of one substance, you can work out the moles, and so the mass, of any other. Practise balancing in how to balance chemical equations.
Common mistakes
- Dividing by atomic mass instead of molar mass when the substance is a molecule: use O₂ = 32, not O = 16.
- Mixing up the number of moles with the number of particles.
- Forgetting the unit g/mol for molar mass.
- Using 22.4 dm³ for a gas at room temperature. Use 24 dm³ unless the question says STP.
- Rounding too early. Keep the full values until the final answer.
Frequently asked questions
What is the definition of a mole in chemistry?
A mole is the amount of a substance that contains exactly 6.02214076 × 10²³ elementary particles, such as atoms, molecules or ions. One mole has a mass in grams equal to the substance's relative formula mass.
What is Avogadro's number used for?
It converts between the number of moles and the number of particles. Multiply the moles by 6.02 × 10²³ to get the number of atoms or molecules, or divide the particles by it to get the moles.
What is the formula for moles?
n = m ÷ M, where n is the number of moles, m is the mass in grams and M is the molar mass in g/mol. For gases at room temperature, n = volume in dm³ ÷ 24.
Why is a mole 6.02 × 10²³?
The number was chosen so that one mole of a substance has a mass in grams equal to its atomic or formula mass. Originally, it was defined as the number of atoms in 12 g of carbon-12. Today the value is fixed exactly.
Try it yourself
Open the Chemistry Formulas tool, choose the moles formula and enter a mass and a molar mass. Check the answer by hand with n = m ÷ M, then use the Molar Mass Calculator to find M for a compound you pick.
See it for yourself, in 3D and free.
Try it in the Chemistry Formulas