# Mole

The amount of a substance that contains 6.02 x 10^23 particles. One mole of a substance weighs its relative formula mass in grams.

Chemistry (school) · Part 05: Moles and calculations · Card 3 of 9 · Last updated 2026-10-19

## Analogy

A mole is like a dozen, but for atoms. A dozen is 12 things. A mole is a huge fixed number of things.

## Example

**One mole weighs**

Carbon (C): 12 g
Water (H2O): 18 g
Sodium chloride (NaCl): 58.5 g

## In practice

One mole of carbon atoms weighs 12 g. One mole of water molecules weighs 18 g.

## Practice

### Flashcards

- **What is the Avogadro constant?** 6.02 x 10^23, the number of particles in one mole.

### Interview questions

**Calculate the mass of 0.5 mol of calcium carbonate, CaCO3. (Ar: Ca = 40, C = 12, O = 16)**

- Mr = 40 + 12 + 3 x 16 = 100
- Mass = moles x Mr
- 0.5 x 100 = 50 g

## Questions people ask

### Why do chemists use moles?

Atoms are far too small to count one by one. The mole links a number of particles to a mass that can be weighed on a balance. The relative formula mass in grams is always one mole, so you can measure out exactly equal numbers of different particles.

### How many atoms are in one mole of carbon?

One mole of carbon contains 6.02 x 10^23 atoms, and weighs 12 g. The same number of atoms is in one mole of any element. So 24 g of magnesium holds the same number of atoms as 12 g of carbon, even though magnesium atoms are heavier.

## Related concepts

- [Avogadro constant](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-4)
- [Moles from mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-5)
- [Relative formula mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-2)

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