Example 1 · level 1
How many moles are present in 192 g of sulphur dioxide (SO₂)? (Atomic masses: S = 32, O = 16)
Show solution
- M(SO₂) = 32 + 2 × 16 = 64 g/mol.
- n = 192 ÷ 64 = 3 mol.
Answer: 3 mol
CLASS 11–12 CHEMISTRY · SOME BASIC CONCEPTS OF CHEMISTRY
Mole concept: mass, moles and particles
Moles n = mass ÷ molar mass. Number of particles = n × Nₐ, where Nₐ = 6.022 × 10²³ mol⁻¹.
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Worked examples
Example 2 · level 2
How many molecules are present in 18.25 g of hydrogen chloride (HCl)? Give the answer as a multiple of 10²³, correct to 2 decimal places (for 3.01 × 10²³ enter 3.01). Take Nₐ = 6.022 × 10²³ mol⁻¹. (Atomic masses: H = 1, Cl = 35.5)
Show solution
- M(HCl) = 1 + 35.5 = 36.5 g/mol.
- n = 18.25 ÷ 36.5 = 0.5 mol.
- Molecules = 0.5 × 6.022 × 10²³ = 3.011 × 10²³ ≈ 3.01 × 10²³.
Answer: 3.01
Example 3 · level 3
How many N atoms are present in 5.6 g of nitrogen gas (N₂)? Give the answer as a multiple of 10²³, correct to 2 decimal places. Take Nₐ = 6.022 × 10²³ mol⁻¹. (Atomic masses: N = 14)
Show solution
- M(N₂) = 2 × 14 = 28 g/mol.
- n = 5.6 ÷ 28 = 0.2 mol of N₂.
- Moles of N atoms = 2 × 0.2 = 0.4 mol.
- Number of N atoms = 0.4 × 6.022 × 10²³ = 2.4088 × 10²³ ≈ 2.41 × 10²³.
Answer: 2.41