Given the balanced equation representing a reaction: 2H2 O2 = 2H2O, what is the total mass of water formed when 8 grams of hydrogen reacts completely with 64 grams of oxygen?

The reaction equation represents the mole number ratio. So the hydrogen is 8/2=4 mol. Oxygen is 64/32=2 mol. So the water is 4 mol. The mass of water is 4*18=72 g. Or if it is confirmed that they are completely reacted. You can simply add them up.