The Mole & the Avogadro Constant (AQA A Level Chemistry): Revision Note
Exam code: 7405
Mole & Avogadro Constant
The Avogadro constant (L) is the number of particles in one mole of a substance
It applies to atoms, molecules, ions, and electrons
The value of L is 6.02 × 1023 mol-1
A mole (mol) is the amount of substance that contains the same number of elementary entities as there are atoms in exactly 12.00 g of carbon-12
One mole of an element has a mass equal to its relative atomic mass (Aᵣ) expressed in grams
One mole of a compound has a mass equal to its relative formula mass (Mᵣ) expressed in grams
For example:
One mole of carbon has a mass of 12 g and contains 6.02 × 1023 carbon atoms.
One mole of water (H2O) has a mass of (2 × 1.0 + 16.0) = 18.0 g and contains 6.02 × 1023 water molecules
The number of moles can be calculated using the equation:
moles = mass ÷ molar mass
The number of particles can then be calculated using:
number of particles = moles × L
Formula triangle diagram linking moles, particles, and the Avogadro constant
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 |
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