# A-level Chemistry/OCR/Atoms, Bonds and Groups/Atoms and Reactions/Moles and Equations

## The Mole

The mole is a unit describing the amount of a substance that you have.

One mole of a substance contains 6.02 x 1023 particles. 6.02 x 1023 is known as Avogadro’s constant.

The molar mass is the mass of one mole: one mole of atoms or molecules has a mass in grams equal to the relative formula mass of that substance. In other words, it is the mass per mole of a substance.

For example: Carbon has an Ar of 12 → 1 mole of carbon weighs 12g → The molar mass of carbon is 12 g/mol.

## Empirical and Molecular Formulae

The empirical formula of a compound is the simplest whole number ratio of an atoms of each element present in a compound.
The molecular formula of a compound gives the actual number of atoms of each element in the compound.

You can calculate the empirical formula by following these steps:

1. Find the mass of each element.
2. Divide the mass by the relative atomic mass (Ar) of the atom/molecule.