**Published on** November 27th, 2012 |
*by Simi*

# Can our family car really be emitting tonnes of greenhouse gases each year? Surely not!

One of the hydrocarbons in petrol is octane C_{8}H_{18}. This burns in oxygen in our engine to form carbon dioxide (a greenhouse gas) and water vapour, as follows:

2C_{8}H_{18} + 25O_{2} → 16CO_{2} + 18H_{2}O.

Using knowledge of relative molecular masses from Chemistry, the above equation tells us that 228 grams of octane C_{8}H_{18} uses 800 grams of oxygen O_{2} to form 704 grams of carbon dioxide CO_{2} and 324 grams of water vapour H_{2}O.

Essentially, the equation tells us that every 228 grams of octane produce 704 grams of carbon dioxide.

The density of octane is close to 700 grams/L, so we can re-write the last statement as:

[latex]\dfrac{228}{700}[/latex] litres of octane cause 704 grams of carbon dioxide to be released into the atmosphere.

The average car travels about 16,000 km each year, and the average consumption for a smaller car (4-cylinder) is about 7.5L/100km.

So this car would use 160 × 7.5 L in the year, or 1,200 L.

[We’re making the simplifying assumption that the fuel is 100% octane, which isn’t so, but it will give us a good estimate of the amount of pollutants. We’re also assuming that we have a perfectly “clean burn”, which also is far from the truth].

**1.** How much carbon dioxide is produced by this small car in a year?

**Extensions**

**2. **Using the fact that 44 grams of CO_{2} occupy 25L of space at normal atmospheric conditions, show that the amount of gas calculated above occupies about 1474m^{3}, which is close to the volume of ten ordinary-sized classrooms. But of course the gas doesn’t stay pure – it spreads out and pollutes everyone’s air.

**3. **Estimate the amount produced in your city for a year, and also for a single day.

Question created by Robert Nelder, Toowoomba State High School

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