What is the Law of Constant Proportions?
The law of constant proportions states that chemical compounds are made up of elements that are present in a fixed ratio by mass. This implies that any pure sample of a compound, no matter the source, will always consist of the same elements that are present in the same ratio by mass. For example, pure water will always contain hydrogen and oxygen in a fixed mass ratio (a gram of water consists of approximately 0.11 grams of hydrogen and 0.88 grams of oxygen, the ratio is 1:8).
The law of constant proportions is often referred to as Proust’s law or as the law of definite proportions. An illustration describing the mass ratio of elements in a few compounds is provided below. The ratio of the number of atoms of each element is provided below the mass ratio. For example, in a nitrogen dioxide (NO2) molecule, the ratio of the number of nitrogen and oxygen atoms is 1:2 but the mass ratio is 14:32 (or 7:16).
In the year 1794, the French chemist Joseph Proust formulated the law of constant proportions from his work on sulfides, metallic oxides, and sulfates. This law was met with a lot of opposition in the scientific community in the 18th century. The introduction of Dalton’s atomic theory favored this law and a relationship between these two concepts was established by the Swedish chemist Jacob Berzelius in the year 1811.
What are the Exceptions to the Law of Constant Proportions?
Despite being a building block in the development of chemistry, the law of constant proportions does not hold true for all chemical compounds. Some exceptions to this law are listed below.
- Some non-stoichiometric compounds have varying compositions of elements between samples. These compounds obey the law of multiple proportions instead.
- One such example is wustite, an oxide of iron with the chemical formula FeO. The ratio of iron and oxygen atoms can range from 0.83:1 to 0.95:1.
- This is caused by the crystallographic vacancies in the samples caused by a disorderly arrangement of atoms.
- Various samples of a compound may vary in the isotopic composition of its constituent elements. This can lead to fluctuations in the mass ratios.
- The differences in the mass ratios between samples are very useful in the process of geochemical dating, due to the preferential concentration of isotopes in many deep Earth and crustal processes.
- This also occurs in many oceanic, atmospheric and even astronomical processes. Despite the effects being quite small, the challenges in the measurement of the effects have been overcome by modern instrumentation.
- Since natural polymers can vary in their compositions, various samples can show different mass proportions.
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