Multiple Proportions, Law of definition of Multiple Proportions, Law of in the Free Online Encyclopedia.

Multiple Proportions, Law of

one of the basic laws of chemistry, which states that if two simple or complex substances form more than one compound with one another, then the masses of one substance required for the same mass of the other substance are related by integers, which are usually small. The law was discovered in 1803 by J. Dalton, who interpreted it from the point of view of atomism. Some examples are given in the following paragraphs.

The composition of nitrogen oxides (in percent by mass) is given in Table 1. Dividing the numbers in the bottom line by 0.57, we find that they are related as 1 : 2 : 3 : 4 : 5.

Table 1. Law of multiple proportions
	Nitrous oxide N₂O	Nitric oxide NO	Nitrous anhydride N₂O₃	Nitrogen dioxide NO₂	Nitric anhydride N₂0₅
N ..........	63.7	46.7	36.8	30.4	25.9
O ..........	36.3	53.3	63.2	69.6	74.1
O/N fraction ..........	0.57	1.14	1.71	2.28	2.85

Calcium chloride forms four hydrates with water; their compositions are given by the formulas CaCl₂-H₂O, CaCl₂-2H₂O, CaCl₂-4H₂O, and CaCl₂-6H₂O— that is, in these compounds, the masses of water required for one molecule of CaCl₂ are related as 1 : 2 : 4 : 6.

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