A mole is the mass of a substance that contains 6.02 x 10²³ particles of that substance. 
Symbol: n
SI unit: mol


Example:
1 mol of carbon-12 has a mass of 12 grams and contains 6.02 x 10²² of carbon atoms. 

To find the number of moles of a substance, we need to use the following formula:
Number of moles = Mass of substance ÷ Mr of molecule
n = M ÷ Mr

In the above equation,

  • Mass is a straightforward quantity measured in grams (g)
  • Mr is the sum of the relative atomic masses (Ar) of each constituent atom present in the molecule. It is measure in arbitrary units or grams per mol (g mol⁻¹). You will be given the Ar of each of the atoms present in the molecule in the exam.

Example:

Find the number of moles in 10 grams of H₂O. 
(Ar of H = 1, O = 16)

We first need to find the Mr of H₂O.
There are two atoms of Hydrogen.
Ar of each atom of H = 1. Ar of two atoms of H = 1 x 2 = 2
There is one atom of Oxygen. Ar of each atom of O = 16. 
Mr of H₂O = 2 + 16 = 18

We are given the mass = 10 g
n = M ÷ Mr
n = 10 ÷ 18
n = 0.5555 ≈ 0.56 mol


You can rearrange the formula to make (M) or (Mr) the subject of the formula. 

  • M = n x Mr
  • Mr = M ÷ n

Question can also involve calculating moles from concentration and volume. If we are given the concentration and volume of a substance, then we can calculate the mole with the following equation:

Moles = Concentration x Volume
n = c x v

Majority of the times, the volume will be given in cm³ and the concentration will be given in mol dm⁻³
You will have to convert the volume given into dm³.

To convert cm³ to dm³, we need to divide the volume by 1000. 
100 cm³ = 0.1 dm³
25 cm³ = 0.025 dm³
50 cm³ = 0.05 dm³


Example:
You are given a 25 cm³ sample of HCl of concentration 0.02 mol dm⁻³. Find the moles of HCl present.

We are given the concentration and the volume of HCl. 
We first need to convert the 25 cm³ to dm³.
25 cm³ = 0.025 dm³

n = c x v
n = 0.02 x 0.025 = 0.0005 mol


You can rearrange the formula to make concentration or volume the subject of the formula. 

  • c = n ÷ v
  • v = n ÷ c