Multiple Proportions, Law of

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 ..........63.746.736.830.425.9
O ..........36.353.363.269.674.1
O/N fraction ..........0.571.141.712.282.85

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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