# 1.3.1 The Mole & the 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

• 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)

• 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

• 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

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