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Law of Constant Proportion

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Last updated date: 22nd Jul 2024
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Chemistry is an important subject on which our life survives. Every basic thing within our body and nature are based on the concept of Chemistry. Chemistry is based on the foundation of the important fundamental laws. The law of conservation of mass, the law of constant proportion, the law of multiple proportions and the law of reciprocal proportion are those main pillars of Chemistry. The law of constant proportion states that in any chemical compound the elements are always present in a constant, definite proportion by mass. This means the same chemical compound will always contain the same elements in the same proportion irrespective of the source, method of preparation of the compound or amount of the compound.

Law of Constant Proportion

To explain the law of constant proportion, it is important to know the law of definite proportions definition. This is what the law states.

The law of constant composition states that the chemical compounds consist of the elements which are present in a fixed ratio by their mass. This means that any given pure sample of the compound, irrespective of its source, will always contain the same kind of elements that are present in the same ratio by the mass. Consider, for example, that pure water would always consist of the hydrogen and oxygen atoms in a mass ratio that is fixed. One gram of water approximately contains 0.11 g hydrogen and 0.88 g oxygen in the ratio of 1:8.

The law of definite proportions is also called Proust's Law. Below is an image to explain the law of constant proportion. It describes the mass ratio of the elements in some compounds. The ratio of the atoms contained in an element is given below as well. Consider, for example, that in the NO2 molecules, the number of nitrogen atoms and the number of oxygen atoms is in the ratio 1:2, however, their mass ratio is found to be 7:16.

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In 1794, Joseph Proust, a French chemist formulated this law of constant proportions from the work he did on sulfates, metal oxides and sulfides. Also, this law was favored since Dalton’s atomic theory was introduced as well. The relation between them was also discovered by Jacob Berzelius, a Swedish chemist in the year 1811.

Now that we know how to define the law of definite proportions, let us take a look at its exceptions.

Exceptions to the Law of Constant Proportion

The law of constant proportion is a building block for the development of Chemistry, however, it does not stand true for all the chemical compounds and has some exceptions. Given below are some of the exceptions to this law.

  1. Some of the non-stoichiometric compounds tend to have varying compositions of the elements between different samples. Hence they tend to follow the law of multiple proportions. 

  2. An example of this is the oxide of iron called wustite, having the formula FeO. The iron and oxygen atoms are in a ratio that ranges from 0.83:1 to 0.95:1. This is because of the crystallographic vacancies present in the samples because of the disordered arrangement of the atoms present.

  3. Different samples of a compound tend to vary in their isotopic composition of the constituent elements. This tends to cause mass ratio fluctuations.

  4. The difference in these mass ratios between the samples are helpful in the geochemical dating because of the preferential concentration of the isotopes in several crustal and deep Earth processes.

  5. This also happens in several atmospheric, astronomical, and oceanic processes. Even if the effects are quite small the challenges faced for the measurement of these effects are overcome by modernly developed instruments.

  6. Since these natural polymers tend to vary in their composition, several samples possess varying mass proportions.

Example of Law of Constant Proportion

  1. Water has both hydrogen and oxygen atoms. One atom of oxygen is combined with two atoms of hydrogen to create the water molecule.

  2. Salt, or NaCl, is composed of Na and Cl atoms. For it to be created, both the sodium and the chlorine atoms have to be in the same proportion.

  3. Sulphuric acid has hydrogen, oxygen and sulfur atoms. All these three atoms have to be in the same proportion for the acid to be created.

FAQs on Law of Constant Proportion

1. State and explain the law of definite proportion.

Let us state the law of constant proportion. 


The law of constant proportion is also referred to as Proust’s law or the law of definite proportion. This law states that any given chemical compound consists of its component elements in a fixed mass ratio. It is not dependent on its method of preparation and its source. Hence, if a compound is made when ¼ of one chemical is combined with the ¾ of the other chemical, these proportions would always hold true irrespective of the amount of the chemicals that are added.


This law of constant proportion makes sure that the chemical compounds are created in a way that they have the same proportions always, irrespective of the amount of the compound that is prepared.

2. What is the law of constant composition?

To understand the law of constant composition, it is essential to state the law of constant proportion. Let us define the law of constant proportion.


The law of constant composition states that in any given chemical compound the elements present in it will always combine in the exact same proportions with one another. 


Now let us state the law of definite proportion in detail with an example.


According to the law of constant composition, all the samples present in the given compound would have the exact same elements in the same mass ratio. Consider, for example, that the water molecules will always have the hydrogen and the oxygen atoms in the ratio 2:1. When we see the relative masses of the oxygen and hydrogen atoms in the water molecule, it is found out that oxygen atoms occupy 94% of the mass and the remaining 6% mass is occupied by the hydrogen atoms. This proportion of the masses remains constant for all the water molecules.

3. Are there any deviations in the law of definite proportion?

This is a fundamental law in the field of Chemistry also referred to as Proust’s law. It states that the ratio of the mass of all the elements present in a compound is always fixed irrespective of its source, method of preparation or amount of the compound. However, there are some deviations from this law and that makes it not to be considered as a universal law. This is because in many non -stoichiometric compounds the elemental composition varies from sample to sample. Such deviations are seen in the following cases:

  • In isotopic composition, the elemental composition may vary depending upon the source. Though the deviation is very less, it can be easily detected in modern-day instruments.

  • In polymers including proteins and DNAs, they deviate from this law.

  • In ores from different sources, the composition of elements present varies thus deviating from the law.

  • Sometimes due to crystallographic vacancies, the formula of a compound varies. For example, FeO may become Fe0.95O.

4. What is the history behind the making of this law?

The Law of Definite Proportion was postulated by the famous French Chemist Joseph Proust. However, the initial observations were made by the English chemist Joseph Priestley and French Chemist Antoine Lavoisier based on their experiments on combustions. Initially, this law, like many other laws, was criticized and debated. However, it was confirmed after the introduction and understanding of the concept of chemical compounds and mixtures. Further, it was confirmed by John Dalton based on his theoretical basis and explanations. According to him, there exists only one type of atom of an element and in a compound, the combinations of atoms of those elements take place in a fixed ratio only.

5. Why are the laws in chemistry important?

Chemistry is a very important subject. Nature, our bodies and all other processes going inside and outside of our body is dependent on Chemistry. It needs some laws to make it easy to understand the reasons behind everything that is taking place in nature. The laws are thus often referred to as the laws of nature. The laws in Chemistry can not only describe the basis of elements and compounds but also the relationship between energy and transformations. The knowledge of laws guides the learner to the world of Chemistry and makes it easy to understand the advanced concepts of Chemistry without any difficulty. 

6. Why is the knowledge of chemistry important?

Chemistry is such an important subject that without it many concepts of other core science subjects can not be understood. That is the reason why subjects like bioChemistry and chemical engineering have found their place in the list of important modern subjects. The making of our body, the different processes running within the body like respiration, digestion excretion and even reading, working all can be explained with Chemistry only.

Career-wise this subject is also valuable. Chemistry opens many opportunities for good career options for the students. Clear concepts in Chemistry can also help the students to get through the major tough competitive exams and get their dream careers.

7. How does vedantu help students  in chemistry?

Vedantu provides rich and resourceful study materials for all subjects. The study materials in Chemistry are prepared by the experienced subject teachers for the benefit of the students. They are available on the Vedantu site and learning app in free downloadable PDFs which can be accessed by anyone. The doubts that arise in the minds of the students can be cleared by one-to-one doubt clearing sessions from subject experts who have years’ experiences. The sample papers and their solutions are also available for the students on their website.