When a substance burns, Oxygen is combined with the original substance so the products are going to be heavier than what was burnt
Answer 2) - Nothing
If you include the weight of the original oxygen that is needed to combine with what is burnt, then the total weight of the ingredients = total weight of the products.
Answer 3) - ???????????
Do you want to quibble about the distinction between mass and weight? I'm not in the mood right now.