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
- 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:
- Na
- H2
- NaCl
Answer 1
- The relative atomic mass of Na is 23.0
- Therefore, 1 mol of Na has a mass of 23.0 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 1.0
- 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 molecules 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 atoms in total
| 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 |
