• 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?N_A?is?6.02 x 10^23?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 10^23?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

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 10^23?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 10^23?molecules of NaCl

Since there are Na and Cl atoms in NaCl, 1 mol of NaCl will contain 1.204 x 10^24?atoms in total