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.
  3. Divide all your answers by the smallest answer.
  4. If required, multiply to make whole numbers.
  5. Use the ratio of atoms to write the empirical formula.

To find the molecular formula from the empirical formula, you need to know the relative formula mass of the compound.



Once the relative formula mass is found, you can divide the molecular formula stated in the question by the relative formula mass, then the answer is multiplied against the empirical formula answer which then gives the new ratio of the atoms in the molecular formula.