Knowledge database: Basic laws of chemistry: Law of multiple proportions

Law of multiple proportions tells us that when we have two elements that combine to give more than one chemical compound, then the masses of one element are in simple ratios with the mass of the second element with which it bonds. In other words, the masses of one element that combines with some other element (which has always the same mass), are in a ratio of small whole numbers.

As an example we can mention the formation of carbon monoxide and carbon dioxide. Both substances consist of oxygen and carbon. 1 gram of carbon always reacts with 1.333 grams of oxygen to form carbon monoxide (CO), while with 2.666 grams of oxygen it forms carbon dioxide (CO₂). As the law says, the first element with the constant mass (1 gram of carbon), merges into two different substances by reacting with oxygen (whose masses are in a ratio of small whole numbers (1:2)).

In a similar way, oxygen and nitrogen form a number of oxides of nitrogen. In this case, if the mass of nitrogen is constant, the masses of oxygen that reacts with nitrogen are also in a ratio of small whole numbers, thus forming different oxides like N₂O, NO, NO₂ and other nitrogen oxides.