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(remember I failed science) ..... but I think along these lines ....... if we had a huge sealed container with ordinary atmospheric air and a car inside ..... started the car ..... the whole container would weigh more after burning the gas

Where would the additional weight come from ? It did not exist at the beginning . To me that is like creating something out of nothing .

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Arnie- Its been decades since my chemistry /physics classes..
but as you mentioned oxygen and all molecules have weight...composed of various combinations of atoms...and numbers thereof.

We are just redistributing combinations of various atomic weights of atoms as molecules are being converted.

No mass is gained or lost....same as no energy is created or lost....the only exception being when God created energy.

You have completed the circle for me and now I understand it a bit better.

And I re-read that article again and realized it said that the carbon in the gallon of gas weighs 5.3 pounds .... but when burned it unites with oxygen and becomes 19 pounds of carbon dioxide in the atmosphere

So would it be correct to say the "carbon portion" still weighs 5.3 lbs but is now "attached" to 13.7 lbs of oxygen making the 19 lb total.

At one time Smokey Yunick was working on an engine that eliminated the CO2. It involved using an engine made with ceramics. Then
one day poof! I haven't heard a thing since.