CIE IGCSE Chemistry

Revision Notes

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:

extended_a table, IGCSE & GCSE Chemistry revision notes

 

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:

extended_b table, IGCSE & GCSE Chemistry revision notes

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.
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