IGCSE Chemistry 0620: Air Composition, Pollutant Sources and Reducing Sulfur Dioxide

Rowanhaven City Council installs an air-quality monitor beside a busy ring road to gather evidence for a proposed low-emission zone. On a still winter morning, the monitor first analyses a 500 dm³ sample of background air away from the traffic to confirm it is essentially clean, dry air.

(a) Calculate the volume, in dm³, of nitrogen gas contained in this 500 dm³ background air sample. [1]

(b) The monitor then analyses fumes collected directly above the traffic queue and identifies raised levels of carbon monoxide and sulfur dioxide. For carbon monoxide, and then for sulfur dioxide, state one source and one adverse effect of the pollutant. [4]

(c) The council wants to reduce the harmful effect of the sulfur dioxide detected. State one strategy, other than the use of catalytic converters, that could reduce sulfur dioxide emissions from vehicles and fuel-burning appliances, and briefly explain how it works. [2]

Part (a): Volume of nitrogen in clean, dry air

Clean, dry air is approximately $78\%$ nitrogen by volume. To find the volume of nitrogen in the $500\ \text{dm}^3$ sample:

$0.78 \times 500 = 390$

So the sample contains $\boxed{390}\ \text{dm}^3$ of nitrogen gas.

Part (b): Sources and effects of carbon monoxide and sulfur dioxide

Carbon monoxide:

Source: produced by the incomplete combustion of carbon-containing fuels, such as petrol or diesel burned in vehicle engines.
Effect: it is a toxic gas (it reduces the blood’s ability to carry oxygen).

Sulfur dioxide:

Source: produced by the combustion of fossil fuels that contain sulfur compounds as an impurity.
Effect: it causes acid rain.

Part (c): Reducing sulfur dioxide emissions

Two strategies (other than catalytic converters, which mainly target oxides of nitrogen) are given in the syllabus for reducing sulfur dioxide emissions:

Using low-sulfur fuels: fuels are treated to remove sulfur compounds before burning, so less sulfur dioxide is formed when the fuel is combusted.
Flue gas desulfurisation: waste gases are passed through calcium oxide (or a calcium oxide/calcium carbonate mixture), which reacts with and absorbs the sulfur dioxide before the gases are released into the atmosphere, preventing it from reaching the air.

Either of these strategies, correctly explained, is an acceptable answer.

Final answers

(a) $390\ \text{dm}^3$ of nitrogen
(b) Carbon monoxide: incomplete combustion of carbon-containing fuels; toxic gas. Sulfur dioxide: combustion of fuels containing sulfur; causes acid rain.
(c) Low-sulfur fuels (less sulfur dioxide formed) or flue gas desulfurisation (calcium oxide absorbs the sulfur dioxide before release).