Knowledge database: 2.4 Law of reciprocal proportions

The law of reciprocal proportions says that the masses of two elemeents 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 mass.

Although at first glance, this law may sound complicated, it is actually easy to understand by examining 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 (1 gram) mass of hydrogen).

In the same way the 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 specific 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 or weights.