The Mole & the Avogadro Constant (Cambridge International AS Chemistry)

The Mole & the Avogadro Constant

• The Avogadro constant(N_A or L) is the number of particles in one mole of a substance

  • The Avogadro constant applies to atoms, molecules, ions and electrons

• The value of N_A is 6.02 x 10²³mol^-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

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
• 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 formula units 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 ions in total (Na⁺ and Cl^–)

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