Example 1 · level 1
How many moles are present in 85 g of ammonia (NH₃)? Atomic masses: N = 14 u, H = 1 u.
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- M = 17 g/mol.
- n = 85 ÷ 17 = 5 mol.
Answer: 5 mol
CLASS 9 SCIENCE · ATOMS AND MOLECULES
Mole concept: mass, moles and particles
moles = mass ÷ molar mass; number of particles = moles × 6.022 × 10²³ (Avogadro constant).
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Worked examples
Example 2 · level 2
How many molecules/formula units are there in 26.5 g of sodium carbonate (Na₂CO₃)? Give the answer as a multiple of 10²³ (e.g. type 12.044 for 12.044 × 10²³). Take Nₐ = 6.022 × 10²³ mol⁻¹. Atomic masses: Na = 23 u, C = 12 u, O = 16 u.
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- n = 0.25 mol.
- N = 0.25 × 6.022 × 10²³ = 1.506 × 10²³.
Answer: 1.506
Example 3 · level 3
A sample contains 18.066 × 10²³ molecules of sulphur dioxide (SO₂). What is its mass in grams (correct to 2 decimal places)? Take Nₐ = 6.022 × 10²³ mol⁻¹. Atomic masses: S = 32 u, O = 16 u.
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- n = 18.066 × 10²³ ÷ 6.022 × 10²³ = 3 mol.
- M = 64 g/mol.
- mass = 3 × 64 = 192 g.
Answer: 192 g