This law was enunciated by Ritcher in 1792-94. It states that “when two different elements separately combine with the fixed mass of third element, the proportion in which they combine with one another shall be either in the same ratio or some simple multiple of it”. For instance, when two elements C and O separately combine with H to form methane (CH4) and water (H2O) respectively it is very clear that in methane 3g of C with 1g of hydrogen, and in water (H2O) 8g of O with the same (fixed) mass i.e (1g) of H. Now when C and O combine with each other to form carbon dioxide (CO2), they do so in the same proportion i.e. 12:32 = 3:8 parts by mass.
Another illustration of law of reciprocal proportion is provided when, 12g of C with 32g of O to form carbon dioxide (CO2) and 32g of sulphur (S) combine with the same (fixed) mass of oxygen (O) i.e. 32g to form sulphur dioxide. The above example, shows that the mass of C and S that with the same mass of O are in the proportion of 12:64 i.e. 3:16.
According to the statement of law of reciprocal proportion, that the proportion in which C and S with one another shall be either in the same ratio (3:8) or same simple multiple of it i.e (3:16).
It is very clear that in the formation of carbon disulphide (CS2), C and S combine in the ratio of (12:64) i.e (3:16) which is simple multiple of (3:8).
