# 4.2.1 The Mole

Extended Only

### The Mole & Avogadro’s Constant

The mole

• This is the mass of a substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of 12C
• The mole is the unit representing the amount of atoms, ions, or molecules
• One mole is the amount of a substance that contains 6.02 x 1023 particles (Atoms, Molecules or Formulae) of a substance (6.02 x 1023 is known as the Avogadro Number)

Examples

• 1 mole of Sodium (Na) contains 6.02 x 1023 Atoms of Sodium
• 1 mole of Hydrogen (H2) contains 6.02 x 1023 Molecules of Hydrogen
• 1 mole of Sodium Chloride (NaCl) contains  6.02 x 1023 Formula units of Sodium Chloride

Linking the mole and the atomic mass

• One mole of any element is equal to the relative atomic mass of that element in grams
• For example one mole of carbon, that is if you had 6.02 x 1023 atoms of carbon in your hand, it would have a mass of 12g
• So one mole of helium atoms would have a mass of 4g, lithium 7g etc
• For a compound we add up the relative atomic masses
• So one mole of water would have a mass of 2 x 1 + 16 = 18g
• Hydrogen which has an atomic mass of 1 is therefore equal to 1/12 the mass of a 12C atom
• So one carbon atom has the same mass as 12 hydrogen atoms
Extended Only

### The Mole & the Volume of Gases

Molar volume

• This is the volume that one mole of any gas (be it molecular such as CO2 or monoatomic such as helium) will occupy
• It’s value is 24dm3 or 24,000 cm3 at room temperature and pressure (r.t.p.)

Calculations involving gases

General equation:

`Amount of gas (mol) = Volume of gas (dm3) ÷ 24`

or

`Amount of gas (mol) = Volume of gas (cm3) ÷ 24000`

1. Calculating the volume of gas that a particular amount of moles occupies

Equation:

`Volume of gas (dm3) = Amount of gas (mol) x 24`

or

`Volume of gas (cm3) = Amount of gas (mol) x 24000`

Example: 2. Calculating the moles in a particular volume of gas

Equation:

`Amount of gas (mol) = Volume of gas (dm3) ÷ 24`

or

```Amount of gas (mol) = Volume of gas (cm3) ÷ 24000

```

Example:  ### Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
Close Close