The Mole & Avogadro Constant (CIE A Level Chemistry)

• The Avogadro constant (N_A 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 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

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 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 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