The mole
Section: 2 Atoms, Molecules and Stoichiometry | Syllabus: Cambridge AS Level Physics 9702
The Mole and the Avogadro Constant The Mole (mol) The SI unit of amount of substance. One mole of any substance contains exactly 6.02 × 10²³ elementary entities (atoms, molecules, ions, electrons, etc.).
This number is the Avogadro constant, Nₐ. Avogadro Constant, Nₐ Nₐ = 6.02 × 10²³ mol⁻¹. The number of specified particles in one mole of a substance. 1 mole of carbon-12 atoms has a mass of exactly 12 g and contains 6.02 × 10²³ atoms.
1 mole of water (H₂O) has a mass of 18 g and contains 6.02 × 10²³ molecules. 1 mole of sodium chloride (NaCl) has a mass of 58.5 g and contains 6.02 × 10²³ formula units. The mole links the macroscopic world (mass in grams) to the atomic world (number of particles).
Key Point The molar mass of a substance (in g mol⁻¹) is numerically equal to its relative atomic mass, relative molecular mass, or relative formula mass. For example, Mᵣ(H₂O) = 18, so molar mass of H₂O = 18 g mol⁻¹.
Key Mole Equations Quantity Formula Units Amount from mass n = m ÷ M n in mol, m in g, M in g mol⁻¹ Number of particles N = n × Nₐ N is dimensionless Mass from moles m = n × M m in g How many moles in 8 g of sulfur (S)?
Aᵣ(S) = 32, so M = 32 g mol⁻¹ n = 8 ÷ 32 = 0.25 mol How many molecules in 0.25 mol of CO₂? N = 0.25 × 6.02 × 10²³ = 1.505 × 10²³ molecules Each CO₂ has 3 atoms, so number of atoms = 3 × 1.505 × 10²³ = 4.52 × 10²³ Figure 2.2: The Mole Triangle (Triangle divided into three sections: top = mass (m), bottom-left = moles (n), bottom-right = molar mass (M).
Arrows showing: n = m ÷ M, m = n × M, M = m ÷ n. A second triangle linking n, N (number of particles), and Nₐ.) Exam Tip Always specify what entity a mole refers to - atoms, molecules, ions, or formula units.
"1 mole of Cl₂" contains 6.02 × 10²³ molecules but 2 × 6.02 × 10²³ atoms.
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