If I use 1 lb of gas to burn 1 lb of wood, how much will the ashes weigh?

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There are too many unknown variables involved to calculate an answer to this question, but I'll try to help you understand the concept.

First of all, if you're referring to using gasoline to burn wood, both are hydrocarbon fuels that undergo combustion. If you're referring to oxygen gas, it's a necessary reactant for combustion of a hydrocarbon fuel.

When a hydrocarbon burns with oxygen, the products are carbon dioxide and water vapor. Both are colorless gases and therefore pretty much invisible. Ashes are left over due to other substances in the fuel that aren't easily combusted, and due to incomplete combustion of the hydrocarbons if adequate oxygen can't get to all of the molecules. If the fuel combusted completely there would be no ash left.

According to the Law of Conservation of Mass, the total mass of products after burning equals the total mass or reactants that were consumed. This doesn't mean that there will be an amount of ash left over that's equal to the mass of the products. Most of the mass of the products is accounted for by the carbon dioxide and water vapor that we know is produced yet we can't see. Similarly, the mass of oxygen gas that reacts accounts for some of the mass of reactants.

Antoine Lavoisier, a French chemist of the 18th century, showed that mass is conserved in a chemical reaction by trapping and weighing the gases produced.

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