And we also have 20 test tubes with stoppers firmly inserted. Here is what you know A cup of coffee costs 50 cents. The point to this type of problem is that it models what you have to do in chemistry. You divide the total weightby the weight per one coin. You counted how many by weighing.
Chemistry does the same thing.
I divide my total mass by the weight of one unit. In the example it is a coin, in chemistry it is the mole. Here's a nice limiting reagent problem we will use for discussion. The lowest number indicates the limiting reagent. Notice that the amount of I 2 does not play a role, since it is in excess.