AQA A Level Chemistry

Revision Notes

1.3.1 The Mole & the Avogadro Constant

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
    • If you had one mole of carbon in your hand, that is 6.02 x 1023 atoms of carbon, you would have a mass of 12.00 g
    • One mole of water would have a mass of (2 x 1 + 16) = 18 g

Worked Example

Moles

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 22.99
    • Therefore, 1 mol of Na has a mass of 22.99 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 1.005
    • Since there are 2 H atoms in H2, the mass of 1 mol of H2 is (2 x 1.005) 2.01 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 1.204 x 1024 H atoms

Answer 3

    • The relative atomic mass of Na and Cl is 22.99 and 35.45 respectively
    • Therefore, 1 mol of NaCl has a mass of (22.99 + 35.45) 58.44 g mol-1
    • 1 mol of NaCl will contain 6.02 x 1023 formula units of NaCl
    • Since there are Na and Cl ions in NaCl, 1 mol of NaCl will contain 1.204 x 1024  ions in total

The Mole Concept Worked Example Table, downloadable IB Chemistry revision notes

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