Gasoline can be approximated by the formula C8H8. An environmental advocate points out that burning one gallon (about 7lbs) of gasoline produces about 19 lb of carbon dioxide, a greenhouse gas that raises the temperature of the atmosphere. Explain this seeming contradiction of the law of conservation of mass

It seems to contradict the law of conservation of mass because matter cannot be created nor destroyed, and in this situation, you started with 7lbs and ended with 19lbs. That would indicate that you created matter..