When methane is burned with oxygen, the products are carbon dioxide and water. if you produce 18 grams of water from 8 grams of methane and 32 grams of oxygen, how many grams of carbon dioxide were produced in the reaction?

The answer is 22.

add up all your reactants which is your oxygen and methane.
32+8=40
now subtract your first product out of that answer

40-28=22

22 is what's left over and the law of consecration of mass means it has to go somewhere. and since we know that methane and oxygen only create carbon dioxide and water we get the answer

there was 22 grams of carbon dioxide produced.