5 Other gases

• Water vapour (H₂O), sulfur dioxide (SO₂), ozone (O₃), carbon monoxide (CO), nitrogen dioxide (NO₂). Several of these are pollutants or reactive trace gases that you study in Modules 10 and 12.

• Problem. A classroom measures 8 m × 6 m × 3 m.
• Assume it is filled with dry air at the composition given above.
• Calculate the volume of each of nitrogen, oxygen, argon and carbon dioxide.
• What does the answer say about our 'safe' limit of CO₂?

Solution. Volume = 8 × 6 × 3 = 144 m³. N₂: 0.78 × 144 = 112.3 m³; O₂: 0.21 × 144 = 30.2 m³; Ar: 0.0093 × 144 = 1.34 m³; CO₂: 0.0004 × 144 = 0.058 m³ = 58 L. A person exhales about 40 000 ppm (4 %) of CO₂; so in a crowded, unventilated room the CO₂ level rises and above about 1000 ppm (0.1 %) people feel drowsy, which is why classrooms need ventilation.

Problem. Compare the amounts of O₂ and CO₂ in the air.

Solution. 21 %/0.04 % = 525; so air has about 500 times as much oxygen as carbon dioxide (the number used in Module 7).