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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 constant

The 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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