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

The law of reciprocal proportions says that the masses of two elements that react with some third element of the same mass, also react with each other, and also with some fourth element with also a certain and constant mass.

Although at first glance, this law may sound complicated, it is actually easy to understand with the help of an example. If 1 gram of hydrogen reacts with 3 grams of carbon thereby forming methane, and if 8 grams of oxygen react with 1 gram of hydrogen thereby forming water, then according to the law that we defined above, 3 grams of carbon can react with 8 grams of oxygen (because both 3 grams of carbon and 8 grams of oxygen react with the same mass (1 gram) of hydrogen).

In the same way, 8 grams of oxygen and 3 grams of carbon can react with some other element (not just the mentioned 1 gram of hydrogen) with a certain and constant mass. Thus, 3 grams of carbon and 8 grams of oxygen can also react with 35.5 grams of chlorine, giving carbon tetrachloride or in the other case, dichlorine monoxide.

The masses of elements with which they enter in mutual chemical reactions are known as equivalent masses.