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Law of Constant Proportions

  • Last Updated : 03 Sep, 2021

We are all aware that matter is made up of atoms. Surprisingly, Greek philosophers proposed the concept of atom in the fifth century BC (BCE). Their notion, however, was philosophical rather than scientific.

John Dalton proposed the first scientific theory of the atom. Few of Dalton’s atomic postulates were proved to be wrong later on in research by J.J. Thomson, Rutherford, Neil’s Bohr, and Schrodinger. Most of the constraints of Dalton’s theory were overcome as a result of the research, and a new hypothesis known as the modern atomic theory was proposed. The following are the major postulates of current atomic theory:

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  • An atom can no longer be divided into two parts.
  • Atoms of the same element can have atomic masses that differ.
  • Atomic masses of multiple elements can be the same.
  • It is possible to convert atoms of one element into atoms of other elements. To put it another way, the atom is no longer unbreakable.
  • Atoms don’t always combine in a straight-forward whole-number ratio.
  • The smallest particle involved in a chemical reaction is the atom.
  • An atom’s mass can be transformed into energy.

What is an Atom?

An atom is the smallest particle of an element that may or may not have an independent existence but is always present in a chemical reaction. An atom is the smallest unit of an element that retains its characteristics. An atom is made up of atoms, which cannot be created or destroyed. Distinct elements have different sorts of atoms, yet all atoms of the same element are identical. When atoms are rearranged, chemical reactions occur.

Atomic Mass and Atomic mass unit: The average mass of an atom (or a set of atoms) is equal to the sum of the masses of the electrons, neutrons, and protons. The atomic mass refers to the mass of a single atom. This is usually stated in terms of a unified atomic mass unit, as stipulated by the international agreement (AMU). The mass is 1/12 that of a carbon-12 atom in its ground state.

What are Molecules?

Molecules are made up of one or more atoms that are held together by chemical bonds. A molecule is the smallest unit of a substance that can still be classified as the same substance. It is made up of two or more atoms that are chemically linked together. 

e.g. Water (H2O), 


  • H2O water
  • N2 nitrogen
  • O3 ozone
  • CaO calcium oxide
  • C6H12O6 glucose
  • NaCl table salt

What is Molecular mass?

The atomic mass of an element is multiplied by the number of atoms in the molecule, and then the masses of all the elements in the molecule are added.

Laws of Chemical Combination

Chemistry is the study of matter’s change from one state to another. The mixing of two different forms of matter frequently results in these alterations. Certain basic laws regulate the mixing of different components to generate compounds. The laws of chemical combination are the name for these rules.

Law of Conservation of mass

This law asserts that neither matter nor energy can be generated or destroyed. To put it another way, the overall mass, which is the sum of the reacting mixture’s mass and the mass of the products generated, remains constant. In 1789, Antoine Lavoisier published this law based on the data he gathered after observing various combustion processes.

Law of Definite Proportions

According to the law of constant composition, chemical compounds are made up of elements that are present in a given ratio based on their mass. This means that any pure sample of the compound, regardless of its source, will always contain the same type of components in the same proportion by mass. Consider the case of pure water, which would always contain hydrogen and oxygen atoms in a set mass ratio. In the ratio 1:8, one gram of water contains approximately 0.11 g hydrogen and 0.88 g oxygen. 

Proust’s Law is another name for the law of definite proportions. In some compounds, it refers to the mass ratio of the elements. Consider that the number of nitrogen atoms and oxygen atoms in NO2 molecules are in a 1:2 ratio, but their mass ratio is 7:16.

From his work on sulphates, metal oxides, and sulphides, Joseph Proust, a French scientist, developed this law of constant proportions in 1794. This law has also been favoured since the introduction of Dalton’s atomic theory. In the year 1811, Jacob Berzelius, a Swedish scientist, identified the link between them.

Pictorial representation of Law of constant proportion

Non-stoichiometric compounds

The law of definite proportions is not universally true, despite its importance in the development of modern chemistry. Non-stoichiometric substances occur, meaning their elemental makeup varies from sample to sample. The law of multiple proportion applies to such compounds. The iron oxide wustite, for example, can have between 0.83 and 0.95 iron atoms for every oxygen atom, resulting in a mass of between 23 and 25 percent oxygen. Due to crystallographic vacancies, the optimum formula is FeO, however it is closer to Fe0.95O. Proust’s measurements were not precise enough to discover such differences in general. Furthermore, an element’s isotopic composition varies depending on its source, thus even a pure stoichiometric compound’s mass contribution can fluctuate. Because astronomical, atmospheric, oceanic, crustal, and deep Earth processes may preferentially concentrate specific environmental isotopes, this variation is employed in radiometric dating. The effect is usually minor, with the exception of hydrogen and its isotopes, but it can be measured with current instrumentation. Even when “pure,” many natural polymers (such as DNA, proteins, and carbohydrates) have different compositions. Except when their molecular weight is consistent (mono-disperse) and their stoichiometry is constant, polymers are not considered “pure chemical compounds.” Due to isotope variances, they may still be breaking the law in this particular circumstance.

Exceptions to the Law of Constant Proportion

The law of constant proportion is a cornerstone of chemistry’s evolution, however it does not apply to all chemical substances and contains several exceptions. Some of the exceptions to this rule are listed below.

  1. The proportions of the components in several non-stoichiometric compounds tend to differ between samples. As a result, they tend to follow the multiple proportions law.
  2. The iron oxide wustite, with the formula FeO, is an example of this. The ratio of iron to oxygen atoms varies between 0.83:1 and 0.95:1. This is due to the crystallographic voids that exist in the samples as a result of the atoms’ disorderly organisation.
  3. The isotopic composition of the constituent elements varies between samples of a substance. This tends to create changes in the mass ratio.
  4. Because of the preferential concentration of isotopes in several crustal and deep Earth processes, the difference in these mass ratios between the samples aids geochemical dating.
  5. This occurs in a variety of atmospheric, astronomical, and oceanic phenomena as well. Even if the impacts are minor, the obstacles of measuring them are addressed by contemporary technologies.
  6. Because the composition of these natural polymers varies, several samples have different mass proportions.

Example of Law of Constant Proportion

  • Both hydrogen and oxygen atoms can be found in water. The water molecule is made up of one atom of oxygen and two atoms of hydrogen.
  • The atoms Na and Cl make up salt, or NaCl. Both the sodium and chlorine atoms must be in the exact same proportion for them to be formed.
  • Hydrogen, oxygen, and Sulphur atoms make up sulphuric acid. For the acid to be formed, all three atoms must be in identical proportion.

Sample Questions

Ques-1 What are the Law of Constant Proportion’s Exceptions?


In non-stoichiometric compounds, the element ratio fluctuates from sample to sample. As a result, the law of constant proportions does not apply to these compounds. Because the masses of two different isotopes of an element differ, samples of elements with varying isotopic compositions can likewise defy the law of definite proportions. Natural polymers have also been shown to defy the proportional law.

Ques-2 Analysis revealed that a 0.24 g sample of oxygen and boron compound contained 0.096 g of boron and 0.144 g of oxygen. Calculate the compound’s % composition based on its mass.


(i) Mass of boron in compound = 0.096 g

And, Mass of compound = 0.24g

So, Percentage of boron

(in compound) = Mass of boron in compound Mass of Compound X =100


= 40

(ii) Mass of oxygen in compound = 0.144 g

And, Mass of compound = 0.24g

So, Percentage of oxygen = Mass of Oxygen in compound Mass of compound X= 100


= 60

Thus, the percentage composition of the compound is : Boron = 40 %; Oxygen = 60 %.

Ques-3 Who was the first to propose the law of definite proportions?


In the year 1779, the French scientist Joseph Louis Proust proposed the law of definite proportions for the first time. This is why the law is also referred to as Proust’s law. The French chemists Antoine Lavoisier and Joseph Priestley were the first to make the discoveries that led to this law.

Ques-4 List some compounds that follow the law of definite proportions.


The hydrogen and oxygen atoms in water molecules are arranged in a 2:1 ratio. Water molecules follow the law of constant proportions because they have a set mass ratio. Methane is another chemical molecule that follows the law of constant proportions. Four hydrogen atoms join with one carbon atom to produce a methane molecule.

Ques-5 Explain and state the Definite Proportion Law.


Proust’s law, or the law of definite proportion, is another name for the law of constant proportion. This law asserts that any chemical compound has a fixed mass ratio of its constituent parts. It is unaffected by the technique of preparation or the source. As a result, if 1/4 of one chemical is coupled with 3/4 of another chemical to make a compound, these proportions will always be true regardless of the amount of chemicals added.

This law of constant proportion ensures that chemical compounds are formed in such a way that their proportions remain consistent no matter how much of the compound is prepared.

Ques-6 A chemical process produces 87 g of carbon dioxide gas from a 150 g baking soda mixture combining sodium bicarbonate and vinegar when heated. How much solid residue will be left in the food?


Total mass of reactants = Total mass of products, according to the law of conservation of mass.

On heating, the baking soda combination (reactant) produces solid residue and carbon dioxide ( products).

On heating, the baking soda combination (reactant) produces solid residue and carbon dioxide ( products).

Mass of baking soda=Mass of solid residue + Mass of carbon dioxide

Hence,  the mass of solid residue is 150g – 87g = 63g.

Ques-7 When it comes to atoms and molecules, what’s the difference?


An atom is a microscopic particle of a chemical element that may or may not exist independently. Molecules are the smallest unit in a complex, consisting of a group of atoms that the bond links together. Chemical bonds exist between two or more identical or different atoms.

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