Here we have a box, just like a compound, this box weighs no less than whatever is inside of it. If you know the weight of all the things inside the box, you know how much it weighs. The reactants in a chemical reaction will form a product and this compound or compounds will have a weight equal to the reactants. In the same way, we figure out the weight of a compound by adding the weight of its atoms. Once we establish a weight for a compound, we can use it for rate calculations. For example, if we know one unit of a product weighs 500g, then 2,000g would be 4 units. If we wanted to know how much 75 units weighs, it would be 75 * 500g which is 37,500g.