Hydrogen and oxygen react chemically to form water. How much water would be formed if 4.8 grams of hydrogen reacted with 38.4 grams of oxygen?
This question is based on the law of conservation of mass. In every chemical and physical change, mass is conserved. Consequently, all we need to do is to add the grams of hydrogen to the rams of oxygen to determine the amount of water that forms, which in this case is 4.8 g H + 38.4 g O = 43.2 g water.
We assume that the reaction goes to completion because, if we look at the ratios, we do have roughly 4.8 moles of H and 2.4 moles of O. In one molecule of water, we have two hydrogen atoms and one oxygen atom, so the proportion fits. The concept of a mole will be explained later, but for this problem we assume it is a way for us to look at proportions between elements.