Sorry bout my '6 gallons' typo.
I see what the problem is in your understanding: You're ignoring the starting weight of the air in that chamber that will soon get combined with the gasoline. Factor that detail in, and it all works.
No it doesn't and here's why.
I'll try explaining once more then I give up.
You have a lawn mower with 1 gallon gas tank. 6 pounds.
Is there a closed container large enough you could put it in, close the door (it's now sealed tight) and run mower until tank is empty. Yes or no?
I'll answer...of course there is.
Maybe it's a 30ft cube, 100ft cube...that's irrelevant. It will run burning up that gallon of gasoline. Yes, of course, running those (6 or 10, whatever) hours that mower has used up a lot of oxygen within container.
Ok. Mower in container WITH gasoline WITH air that's in it TOTAL weighs (again, weight is irrelevant...say it's 10,000 pounds). When that mower cuts off, all gas has run out...it IS NOT going to now weigh 10,020 pounds!
I'm no scientist, engineer, chemist but have common sense!
Six pounds of gasoline does not produce 20 pounds of CO2 air...that's pure bs. The closed container makes it easy to visualize.
You cannot put 6 pounds of gasoline in a hanging on a scale container, then ignite it by match or engine and watch that scale increase 20 pounds!!! No way, regardless of who says that!
