The law of reciprocal proportions, also called law of equivalent proportions or law of permanent ratios, is one of the basic laws of

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

. It relates the proportions in which elements combine across a number of different elements. It was first formulated by Jeremias Richter in 1791. A simple statement of the law is: :If element A combines with element B and also with C, then, if B and C combine together, the proportion by weight in which they do so will be simply related to the weights of B and C which separately combine with a constant weight of A. :As an example, 1 gram of

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

(Na = A) is observed to combine with either 1.54 grams of

chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...

(Cl = B) or 5.52 grams of

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

(I = C). (These ratios correspond to the modern formulas NaCl and NaI). The ratio of these two weights is 5.52/1.54 = 3.58. It is also observed that 1 gram of chlorine reacts with 1.19 g of iodine. This ratio of 1.19 obeys the law because it is a simple fraction (1/3) of 3.58. (This is because it corresponds to the formula ICl₃, which is one known compound of iodine and chlorine.) Similarly, hydrogen, carbon, and oxygen follow the law of reciprocal proportions. The acceptance of the law allowed tables of element equivalent weights to be drawn up. These equivalent weights were widely used by chemists in the 19th century. The other laws of stoichiometry are the

law of definite proportions In chemistry, the law of definite proportions, sometimes called Proust's law or the law of constant composition, states that a given chemical compound contains its constituent elements in a fixed ratio (by mass) and does not depend on its source ...

and the

law of multiple proportions In chemistry, the law of multiple proportions states that in compounds which contain two particular chemical elements, the amount of Element A per measure of Element B will differ across these compounds by ratios of small whole numbers. For inst ...

. The law of definite proportions refers to the fixed composition of any compound formed between element A and element B. The

describes the stoichiometric relationship between two or more different compounds formed between element A and element B. The law states that if two different elements combine separately with a fixed mass of a third element, the ratio of the masses in which they combine are either the same or are in simple multiple ratio of the masses in which they combine with each other .

History

The law of reciprocal proportions was proposed in essence by Richter, following his determination of neutralisation ratios of metals with acids. In the early 19th century it was investigated by Berzelius, who formulated it as follows: :When two substances, A and B have an affinity for two others, C and D, the ratio of the quantities C and D which saturate the same amount of A is the same as that between the quantities C and D which saturate the same amount of B. Later Jean Stas showed that within experimental error the stoichiometric laws were correct.

References

{{reflist Stoichiometry