The Mole & the Avogadro Constant (AQA A Level Chemistry)

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Mole & Avogadro Constant

  • The Avogadro constant (NA or L) is the number of particles equivalent to the relative atomic mass or molecular mass of a substance
    • The Avogadro constant applies to atoms, molecules, ions and electrons
  • The value of NA is 6.02 x 1023 g mol-1
  • The mass of a substance with this number of particles is called a mole (mol)
    • The mass of a substance containing the same number of fundamental units as there are atoms in exactly 12.00 g of 12C
  • One mole of any element is equal to the relative atomic mass of that element in grams
    • One mole of carbon, that is if you had 6.02 x 1023 atoms of carbon in your hand, would have a mass of 12 g
    • One mole of water would have a mass of (2 x 1 + 16) = 18 g
  • The number of moles or particles can be calculated easily using a formula triangle:

Formula triangle diagram linking moles, particles and Avogadro's constant 

Formula triangle diagram linking moles, particles and Avagadro's constantThe moles and particles formula triangle – cover the one you want to find and follow the directions in the triangle

Worked example

Determine the number of atoms, molecules and the relative mass of 1 mole of:

  1. Na
  2. H2 
  3. NaCl

Answer 1

  • The relative atomic mass of Na is 23.0
  • Therefore, 1 mol of Na has a mass of 23.0 g mol-1
  • 1 mol of Na will contain 6.02 x 1023 atoms of Na (Avogadro’s constant)

Answer 2

  • The relative atomic mass of H is 1.0
  • Since there are 2 H atoms in H2, the mass of 1 mol of H2 is (2 x 1.0) = 2.0 g mol-1
  • 1 mol of H2 will contain 6.02 x 1023 molecules of H2
  • Since there are 2 H atoms in H2, 1 mol of H2 will contain 2 x 6.02 x 1023  = 1.204 x 1024 H atoms

Answer 3

  • The relative atomic mass of Na and Cl is 23.0 and 35.5 respectively
  • Therefore, 1 mol of NaCl has a mass of (23.0 + 35.5) = 58.5 g mol-1
  • 1 mol of NaCl will contain 6.02 x 1023 molecules of NaCl
  • Since there is one Na and one Cl atom in NaCl, 1 mol of NaCl will contain 2 x 6.02 x 1023  = 1.204 x 1024 atoms in total
1 mole of  Number of atoms Number of molecules Relative mass (g mol-1)
Na 6.02 x 1023 - 23.0
H2 1.204 x 1024 6.02 x 1023 2.0
NaCl 1.204 x 1024 6.02 x 1023 58.5

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.