The Law of Multiple Proportions states that when two different elements combine to form more than one compound, the quantity of one element combined with a fixed amount of the other element, exhibits a simple whole number ratio.
For example, in nitric oxide (NO) and nitrogen dioxide (NO2), the element nitrogen combines with oxygen in different proportions. A fixed mass of nitrogen, say 14 grams, may react with 16 grams of oxygen to produce nitric oxide (NO), or with 32 grams of oxygen to produce nitrogen dioxide (NO2). Thus, the mass ratio of oxygen which can react with nitrogen is 32:16, which can be further simplified to simple whole numbers:
32:16 = 2:1
The 2:1 mass ratio of oxygen suggests that for nitric oxide (NO) or nitrogen dioxide (NO2), one nitrogen atom is associated with either one or two oxygen atoms, respectively. Thus, understanding the Law of Multiple Proportions is fundamental for understanding stoichiometry, as well as differentiating between different compounds composed of the same elements.