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