Law of Definite Proportion | Definition, History, Examples

Definition of Law of Definite Proportion

Law of definite proportion, states that every chemical compound contains fixed and constant proportions (by mass) of its constituent elements.

law of definite proportion

Other Definitions

  1. Although many experimenters had long assumed the truth of the principle in general. The French chemist Joseph-Louis Proust first accumulated conclusive evidence for it in a series of researches on the composition of many substances, especially the oxides of iron (1797).
  2. Another French chemist, Claude Berthollet, who held for indefinite proportions, contested Proust’s findings. But the Scottish chemist Thomas Thomson confirmed some of them and wrote in his article “Chemistry” in the Supplement to the Encyclopedia Britannica (1801) that Proust had definitely proved “metals are not capable of indefinite degrees of oxidation.
  3. ” The principle was then concretely formulated by the English chemist John Dalton in his chemical atomic theory (1808). The law of constant proportions is often referred to as Proust’s law or as the law of definite proportions. The law of constant proportions states that chemical compounds are made up of elements present in a fixed ratio by mass. This indicates that any pure sample of a compound, no matter the source, will always comprise of the same elements that are present in the same ratio by mass.
  4. The discovery that mass was always conserved in chemical reactions was soon followed by the law of definite proportions, which states that a given chemical compound always contains the same elements in the same proportions by mass. This implies that any pure sample of a compound, no matter the source, will always consist of the same elements present in the same ratio by mass.

History of Law of Definite Proportion

Joseph Proust gave the law of definite proportion in 1797. This observation was first made by the English theologian and chemist Joseph Priestley and Antoine Lavoisier, a French nobleman and chemist centred on the process of combustion.

I shall conclude by deducing from these experiments the principle. I have established at the commencement of this memoir, viz. that iron, like many other metals, is subject to the law of nature which presides at every true combination. That is to say, that it unites with two constant proportions of oxygen.

In this respect, it does not differ from tin, mercury, and lead and, in a word, almost every known combustible. The law of definite proportions might seem obvious to the modern chemist, inherent in the very definition of a chemical compound.

At the end of the 18th century, however, the law was novel when the concept of a chemical compound had not yet been fully developed. In fact, when first proposed, it was a controversial statement and was opposed by other chemists, most notably Proust’s fellow Frenchman Claude Louis Berthollet, who argued that the elements could combine in any proportion.

The existence of this debate demonstrates that, at the time, the distinction between pure chemical compounds and mixtures had not yet been fully developed.

The law of definite proportions contributed to and was placed on a firm theoretical basis by the atomic theory that John Dalton promoted beginning in 1803. Which explained the matter as consisting of discrete atoms. That there was one type of atom for each element, and that the compounds were made of combinations of different types of atoms in fixed proportions.

Applications of the Law of Definite Proportion

  • The law of definite proportion has applications to both molecular compounds with a fixed composition and ionic compounds. As they require certain ratios to achieve electrical neutrality.
  • The law of definite proportions dictates that a name is always associated with a specific ratio of elements found in a chemical compound. If the ratio of elements is different from that specific ratio, then it is not the same compounds and therefore has a different name.

Exceptions to the Law of Definite Proportions

Despite being a building block in the development of chemistry. The law of definite proportions does not hold true for all chemical compounds. Some exceptions to this law are listed below

  1. Some non-stoichiometric compounds have varying compositions of elements between samples. These compounds obey the law of multiple proportions instead.
  2. 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.
  3. This is caused by the crystallographic vacancies in the samples caused by a disorderly arrangement of atoms.
  4. Various samples of a compound may vary in the isotopic composition of its constituent elements. This can lead to fluctuations in the mass ratios.
  5. The differences in the mass ratios between samples are very useful in geochemical dating. Due to the preferential concentration of isotopes in many deep Earth and crustal processes.
  6. 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.
  7. Since natural polymers can vary in their compositions. Various samples can show different mass proportions.

Examples of Law of Definite Proportion

Here are the some examples of law of definite proportion:

examples of law of definite proportion


  • Any sample of pure water contains 11.19% hydrogen and 88.81% oxygen by mass. It does not matter where the sample of water came from or how it was prepared. Its composition, like that of every other compound, is fixed.


  • Another example is carbon dioxide. This gas is produced from a variety of reactions, often by the burning of materials. The structure of the gas consists of one atom of carbon and two atoms of oxygen. Carbon dioxide production is of interest in many areas, from the amount we breathe to the amount of gas produced by burning wood or fossil fuels. By knowing the exact composition of carbon dioxide, we can predict the effects of different chemical processes.


  • In a nitrogen dioxide (NO2) molecule, the number of nitrogen and oxygen atoms is 1:2, but the mass ratio is 14:32 (or 7:16).


  • Salt, written as the chemical compound NaCl, is made up of Sodium (Na) and Chlorine (Cl) atoms. The same proportion of sodium and chlorine must always be combined in order for salt to be created.


  • Sulfuric acid is made up of the individual elements of hydrogen, sulphur, and oxygen. The chemical compound is written H2SO4. The same proportions of hydrogen, sulphur, and oxygen must be combined to create sulfuric acid.


  • Ammonia is a common household item made up of the elements hydrogen and nitrogen. It is written as NH3, meaning that there is one atom of nitrogen combined with 3 atoms of hydrogen. Anhydrous ammonia contains 82% nitrogen and 18% hydrogen. Any other combination of hydrogen and nitrogen would result in an entirely different chemical compound.


  • Vinegar is another common household item. And its chemical compound is C2H4O2, meaning that it is made of 2 atoms of carbon, 4 atoms of hydrogen, 2 atoms of oxygen. No other combination of carbon, hydrogen, and oxygen could be combined to create vinegar.


  • Glucose is a chemical compound that is written C6H12O6. This means that for a substance be glucose, and it must be comprised of 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen. Carbon makes up 40.001% of glucose; hydrogen makes up 6.714% of glucose, and oxygen makes up 53.285% of glucose. It wouldn’t work if you tried to create glucose with the carbon and oxygen equally represented by combining 45% carbon, 45% oxygen, and 10% hydrogen; the chemical compound that would result would not be glucose.

Also Read –

Leave a Comment