CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.2.2 The Mole & 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
  • 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

Worked Example: Moles

Atoms, Molecules & Stoichiometry Worked Example - Moles, downloadable AS & A Level Chemistry revision notes

Answer 1

The relative atomic mass of Na is 22.99 g mol-1

Therefore, 1 mol of Na has a mass of 22.99 g

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 g mol-1

Since there are 2 H atoms in H2, the mass of 1 mol of H2 is (2 x 1.005) 2.01 g

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 g mol-1 and 35.45 g mol-1 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 molecules of NaCl

Since there are Na and Cl atoms in NaCl, 1 mol of NaCl will contain 1.204 x 1024 atoms in total

Mole & Avogadro Constant Worked Example table, downloadable AS & A Level Chemistry revision notes

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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