Which mass of oxygen completely reacts with 4.0 grams of hydrogen to produce 36.0 grams of water

This problem may easily solved by applying the conservation of mass, which states that the total mass before and after a change is constant because mass can neither be created nor destroyed.

We know that water consists of only hydrogen and water, and that there are no other reactants except hydrogen and oxygen. Thus:

Mass of reactants = Mass of product

mass of hydrogen + mass of oxygen = mass of water

4 + mass of oxygen = 36

mass of oxygen = 32 grams

Answer is 32.0 Gram basically its like 36 grams - 4 grams