Mole & 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:
- 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 1.005
- 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 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