The Mole
- The Avogadro constant (NA or L) is the number of particles equivalent to the relative atomic mass or molecular mass of a substance in grams
- The Avogadro constant applies to atoms, molecules and ions
- The value of the Avogadro constant is 6.02 x 1023 mol-1
- The mass of a substance with this number of particles is called the molar mass
- One mole of a substance contains the same number of fundamental units as there are atoms in exactly 12.00 g of 12C
- If you had 6.02 x 1023 atoms of carbon-12 in your hand, you would have a mass of exactly 12.00 g
- One mole of water would have a mass of (2 x 1.01 + 16.00) = 18.02 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 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 1.01
- Since there are 2 H atoms in H2, the mass of 1 mol of H2 is (2 x 1.01) 2.02 g mol-1
- 1 mol of H2 will contain 6.02 x 1023 molecules of H2
- However, since there are 2 H atoms in each molecule of H2, 1 mol of H2 molecules will contain 1.204 x 1024 H atoms
Answer 3:
- The relative atomic masses of Na and Cl are 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 is both an Na and a Cl atom in NaCl, 1 mol of NaCl will contain 1.204 x 1024 atoms in total
Summary:
| 1 mole of | Number of atoms | Number of molecules/ formula units | Relative mass |
| Na | 6.02 x 1023 | - | 23.99 |
| H2 | 1.204 x 1024 | 6.02 x 1023 | 2.02 |
| NaCI | 1.204 x 1024 | 6.02 x 1023 | 58.44 |
