About the Law of Multiple Proportions


Chemistry is a relatively new science that did not exist in ancient times. Pioneers, such as John Dalton, helped chemistry develop and grow almost entirely within the past 100 years. In the early 1800s, Dalton unveiled his law of multiple proportions, which explains how elements unite. According to the law, when two elements form multiple compounds, the masses of an element that joins with 1 gram of another element can be expressed as a ratio that's always a small whole number. Implications of this law are significant in chemistry and the study of atomic theory.

Chemistry: The Evolution

  • As late as the beginning of the last century, knowledge about chemistry was limited. No chemical laws existed, and scientists didn't know that substances had fixed compositions. In 1801, Proust discovered that substances combine in specific ratios by weight -- a fact that modern chemistry students learn quickly. A few years later, in 1804, John Dalton revealed his law of multiple proportions and brought the concept of atomic theory into the fledgling science of chemistry. After creating his Table of Atomic Weights, he wrote about his theory in the book "A New System of Chemical of Philosophy."

Prove the Law

  • A real-world example can help you visualize the law of multiple proportions. Consider dinitrogen monoxide and nitrogen dioxide. Nitrogen and oxygen combine to form each of these compounds, but their mass ratios differ. Dinitrogen monoxide has 0.570 grams of oxygen for every 1 gram of nitrogen. You'll find 2.28 grams of oxygen for every 1 gram of nitrogen in nitrogen dioxide. In this example, nitrogen is the element that has a fixed mass of 1. When you divide the mass of oxygen in nitrogen dioxide, 2.28 by 0.570, the mass of oxygen in dinitrogen monoxide, you get 2.28/0.570 or 4 -- a whole number that proves the law of multiple proportions.

Dalton's Reasoning

  • Don't confuse the law of multiple proportions with the law of definite proportions, which simply states that the ratio of one element to another in a compound is always the same. These laws may differ, but they help confirm the atomic nature of matter. Dalton's law of multiple proportions makes sense when you consider the indivisible nature of atoms. For example, when you combine one atom of element A with one atom of element B in one compound, or two atoms of element B in another compound, etc., it's impossible for the ratios of element B in those compounds to be a fraction.

Confirming the Law of Multiple Proportions

  • When Dalton presented his law, he did not offer any single standard for computing atomic weights. His theory also didn't distinguish between molecules and atoms the way modern chemistry does. In 1808, William Hyde Wollaston and Thomas Thomson confirmed Dalton's law of multiple proportions. They did this by comparing two salts formed when you combine oxalic acid, strontia and potash. In 1812, Jons Jacob Berzelius performed a significant number of analysis to confirm Dalton's law of multiple proportions.


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