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law of simple multiple proportions, in chemistry, the statement that when two or more elements form more than one compound, the ratio of the weights of one element that combine with a given weight of another element in the different compounds is a ratio of small whole numbers. For example, carbon and oxygen combine in carbon dioxide (CO2) and carbon monoxide (CO). A sample of carbon dioxide containing 1 gram of carbon contains 2.66 grams of oxygen; a sample of carbon monoxide containing 1 gram of carbon contains 1.33 grams of oxygen. The ratio of the two weights of oxygen (2.66:1.33) is exactly 2:1. The law of simple multiple proportions can be regarded as an extension of the early law of definite composition, which states that the proportions by weight of the elements present in any pure compound are always the same. An even broader generalization is the law of combining (or equivalent) weights (also known as the law of reciprocal proportions), which states that the ratio in which two substances react with each other is the ratio, or some multiple of the ratio, of the weight of the same two substances reacting with a third substance. All three laws are elementary consequences of the atomic theory, as proposed by John Dalton (see atom; chemistry).
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