What is the amount of carbon dioxide emitted by burning one pound of fossil fuels?

First, it should be understood that the gallon of gas will not create any carbon. All the carbon due to the gallon of gas is already there, in and part of the gas(oline). It amounts to about 12/14 of the gasoline, so the carbon content of a gallon of gasoline is 12/14 of 6.2 lb (wt of a gallon of gasoline), or about 5.3 lb of carbon. It will still be 5.3 lb of carbon even if you burn it, since matter is not destroyed in a chemical reaction.

If the question is how much carbon dioxide, the simplest answer is that there is about 3.2 times as much carbon dioxide produced in burning alkanes (like octane) as the weight of the alkane.

A gallon of gas weighs about 6.2 lb and will combine with oxygen to make about 3.2x6.2 or 20 lb of CO2.