You have to have the right amount of ingredients to produce the amount of food you want. For example, you're going to need at least a tablespoon of salt in a cookie dough recipe of you want the cookies to be salty. In chemistry, you need a specific amount to produce a product. For example, you will need 1 atom of Chlorine and 1 atom of Sodium in order to to produce one molecule of NaCl (table salt).