Burning of petrol to Carbon dioxide

[sigma]

How can a gallon of petrol with a density of 7lb produce, when burnt in a car engine, 12 lb of carbon dioxide.

[gen2]

The density of petrol is less than 1
Density has nothing at all to do with this question.
When a fuel burns, it combines with oxygen from the air.

[eltelioni]

Petrol is made up of hydrocarbons. I guess about 85% of the mass is carbon. Each carbon atom burns to make one molecule of CO2. The mass of a CO2 molecule is approximately 2.3 times the mass of the carbon atom, so the CO2 produced from 7lb of petrol will be about:

7lb x 85% * 2.3 = 13.7lb

Ok, maybe my figures aren't very accurate but it is in the right ballpark.

[sigma]

Thanks

[dundurn]

Atomic weight of carbon is 12, oxygen 16, so molecule of CO2 is 44. This gives a ratio of 44/12 which is 3.67 to 1. So I would revise eltelioni's figure to 21.8lb.

Some petrol remains unburnt and some goes to carbon monoxide but I wouldn't have thought this would have amounted to a reduction of almost 10lb. Are you sure the 12lb figure is correct?

[eltelioni]

Oops. To make up for my silly error I did some research.
1 US gallon of petrol weighs 6lb.
1 UK gallon of petrol weighs 7.5lb
petrol is 87% carbon (by weight)
99% of the carbon in petrol burns to produce CO2

So, in the US, 1 gallon petrol burns to:
6lb x 87% x 99% x 3.67 = 19lb CO2

In the UK:
7.5lb x 87% x 99% x 3.67 = 23.7lb CO2

According to WikiAnswers the CO2 weights are 19.4lb (US) and 24.25lbs (UK)