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Law of Constant Proportions

Last Updated : 22 Apr, 2024
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Law of Constant Proportions, also known as the Law of Definite Proportions, is a fundamental principle in chemistry that states that a given chemical compound always contains its constituent elements in fixed and definite proportions by mass, regardless of its source or method of preparation. This means that the ratio of the masses of the elements in a compound is always constant and does not change under normal chemical reactions or physical conditions.

In this article, we will learn in detail about law of constant proportions and its examples.

Law of Conservation of Mass

A Mass cannot be generated or destroyed in an isolated system, but it can be converted from one form to another.

The mass of the reactants must equal the mass of the products in a low-energy thermodynamic process, according to the law of conservation of mass. It’s thought that mass conservation is defined by a few assumptions from classical mechanics. With the help of quantum mechanics and special relativity, the law of conservation of mass was later amended to the point where energy and mass are now one conserved quantity. The conservation of mass was discovered by Antoine Laurent Lavoisier in 1789.

Formula of Law of Conservation of Mass

In fluid mechanics and continuum mechanics, the law of conservation of mass can be stated in differential form using the continuity equation as:

∂ρ∂t +▽ (ρv) = 0


  • ρ is the density,
  • t is the time,
  • v is the velocity, and
  • ▽ is the divergence.

Examples of Law of Conservation of Mass 

  • Combustion process: Burning of wood is a conservation of mass as the burning of wood involves Oxygen, Carbon dioxide, water vapour and ashes.
  • Chemical reactions: To get one molecule of H2O water with the molecular weight of 10, Hydrogen with molecular weight 2 is added with Oxygen whose molecular weight is 8, thereby conserving the mass.

The Law of Conservation of Mass-Energy

The law of mass-energy conservation, which states that the total mass and energy of a system remain constant. The knowledge that mass and energy can be converted from one to the other is incorporated in this revision. because the amount of energy produced or used in a normal chemical reaction is so small In a reaction, the total number of atoms stays the same. 

This assumption allows us to formulate a chemical reaction as a balanced equation, in which both sides of the equation have the same number of moles of each element. Another significant application of this law is determining the masses of gaseous reactants and products. Any residual mass can be attributed to gas if the sums of the solid or liquid reactants and products are known.

Although it may appear like burning destroys matter, the same amount (or mass) of the matter remains after a campfire. When wood burns, it combines with oxygen and transforms into ashes, carbon dioxide, and water vapour, among other things. The gases float away into the air, leaving only the ashes behind.

What is Law of Constant Proportions?

According to the law of constant proportions, chemical compounds are made up of elements that are present in a stable mass ratio. This means that regardless of the source, each pure sample of a chemical will always have the same elements in the same mass ratio. 

Pure water, for example, will always have a constant mass ratio of hydrogen to oxygen (a gram of water consists of approximately 0.11 grams of hydrogen and 0.88 grams of oxygen, the ratio is 1:8). Chemical compounds are composed up of elements that have a constant mass ratio, according to the law of constant proportions. This indicates that any pure sample of a chemical will always have the same elements in the same mass ratio, regardless of the source.

From his work on sulphides, metallic oxides, and sulphates, the French scientist Joseph Proust created the law of constant proportions in 1794. In the 18th century, this regulation was faced with a lot of hostility from the scientific community. The introduction of Dalton’s atomic theory favoured this law, and the Swedish chemist Jacob Berzelius demonstrated a relationship between the two notions in 1811.

Exceptions to the Law of Constant Proportions

The law of constant proportions does not apply to all chemical substances, despite its importance in the evolution of chemistry. This law has a few exceptions, which are described below.

  • The composition of components in some non-stoichiometric compounds varies from sample to sample. Instead, the law of multiple proportions governs these compounds.
  • Wustite, an iron oxide with the chemical formula FeO, is one such example. The proportion of iron to oxygen atoms can vary between 0.83 and 0.95.
  • This is due to crystallographic voids in the samples, which are created by a chaotic atom arrangement.
  • The isotopic composition of a compound’s constituent elements may differ between samples. The mass ratios may fluctuate as a result of this.
  • Due to the preferential concentration of isotopes in many deep Earth and crustal processes, changes in mass ratios across samples are highly valuable in the process of geochemical dating.
  • Many marines, atmospheric, and even celestial processes are affected by this. Despite the fact that the impacts are minor, modern instrumentation has solved the obstacles of measuring them.
  • Because natural polymers have a wide range of compositions, different samples may have varied mass proportions.

Dalton’s Law

In his 1804 publication, A New System of Chemical Philosophy, English chemist and meteorologist John Dalton introduced the concept of multiple proportions, often known as Dalton’s law. It’s a stoichiometric rule. The law asserts that when elements form compounds, the proportions of the components in those chemical compounds can be stated in small whole-number ratios. It was based on Dalton’s observations of atmospheric gas reactions.

The atoms carbon and oxygen, for example, can react to produce both carbon monoxide (CO) and carbon dioxide (CO2) (CO2). The amount of oxygen compared to the amount of carbon in CO2 has a fixed ratio of 1:2, which is a ratio of whole numbers. The ratio in CO is 1:1.

Dalton proposed the idea that all matter is made up of diverse combinations of atoms, which are the indivisible building blocks of matter, in his theory of atomic structure and composition. These rules lay the foundation for our current understanding of atomic structure and composition, as well as concepts such as molecular and chemical equations.


As we explore the vast world of chemistry, there’s this super important thing called the Law of Constant Proportions. It’s like a timeless rulebook that helps us understand how compounds are made and what they’re made of. Whether we’re talking about tiny molecules or huge things like stars and planets, this rule shows us that there’s a consistent pattern in how stuff comes together. It’s like a reminder of the order that exists in everything around us, making us appreciate the incredible complexity and beauty of our universe.

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Sample Questions

Question 1: State the Law of Conservation of Mass and Energy.


Although mass and energy cannot be converted, their total remains constant during any physical or chemical transformation.

The mass of the products in a nuclear reaction is slightly less than that of the reactants. The reason for this is that the lost mass is transformed into energy using the following equation:


m = mass lost

c = velocity of light

Question 2: If 4.2 g of KClO3 is heated, the result is 1.92 g of O and 2.96 g of KCl. Demonstrate that this result follows the Mass Conservation Law.


KClO3 →  KCl + 3/2 O2

Sum of masses of products

1.92  +  2.96  =  4.88 g

The difference between the total mass of reactants and products and the sum of products is calculated as follows:

0.02 g = 4.9 – 4.88

Question 3: The value must have been zero in this case; yet, for experimental errors, the law of conservation of mass still applies.

What is the Ultimate Source of Energy if it can neither be created nor destroyed?


The Big Bang is the source from which we obtain energy. At the beginning of time, all energy was produced. As the universe expanded, it created a variety of materials, which in turn produced energy.

Question 4: 5.3 g sodium carbonate and 6 g ethanoic acid were combined in a process. 2.2 g carbon dioxide, 0.9 g water, and 8.2 g sodium ethanoate were the end products. Demonstrate that these findings are consistent with the law of mass conservation.

Sodium ethanoate + carbon dioxide + water sodium carbonate + ethanoic acid sodium ethanoate + carbon dioxide + water


Sodium carbonate interacts with ethanoic acid to generate sodium ethanoate, carbon dioxide, and water in the given reaction. 5.3 gram of sodium carbonate (Given)

Mass of sodium carbonate = 5.3 g (Given)

Mass of ethanoic acid = 6 g (Given)

Mass of sodium ethanoate = 8.2 g (Given)

Mass of carbon dioxide = 2.2 g (Given)

Mass of water = 0.9 g (Given)

Now, total mass before the reaction = (5.3 + 6) g= 11.3 g

After the reaction, the total mass is (8.2 + 2.2 + 0.9) g.(11.3 g)

Before the reaction, the total mass was = the total mass after the reaction.

As a result, the observations are consistent with the law of mass conservation.

Question 5: The law of conservation of mass results in which assumption of Dalton’s atomic theory?


‘The relative number and types of atoms are constant in a given compound,’ according to Dalton’s atomic theory, which is based on the rule of conservation of mass. In a chemical reaction, atoms cannot be generated or destroyed.’

Postulates of Dalton’s atomic theory

  1. Atoms are the indestructible building blocks of matter..
  2. All of the atoms of a given element have the same properties, including mass. This can alternatively be expressed as an element’s atoms all having the same mass, whereas other elements have varying masses.
  3. Compounds are formed when atoms of different elements mix in specific proportions.
  4. Atoms do not make or destroy themselves. This means that no atoms are generated or destroyed during chemical processes.
  5. The rearrangement of existing atoms results in the production of new products (compounds) (reactants).
  6. The mass, size, and many other chemical and physical properties of an element’s atoms are all the same.
  7. Even so, the mass, size, and many other chemical and physical properties of atoms from two different elements differ.

Question 6: Water is formed when hydrogen and oxygen mix in a mass ratio of 1:8. How much oxygen gas would it take to totally react with 3 g of hydrogen gas?


It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8.

Then, the mass of oxygen gas required to react completely with 1 g of hydrogen gas is 8 g.

Therefore, the mass of oxygen gas required to react completely with 3 g of hydrogen gas is 8 × 3 g = 24 g.

Question 7: The law of conservation of mass results in which assumption of Dalton’s atomic theory?


The following is the postulate of Dalton’s atomic theory, which is based on the rule of mass conservation:

In a chemical reaction, atoms are indivisible particles that cannot be formed or destroyed.

FAQs on Law of Constant Proportions

What does the Law of Constant Proportions state?

The Law of Constant Proportions asserts that a given chemical compound always contains its constituent elements in fixed and definite proportions by mass. Regardless of the source or method of preparation, the ratio of the masses of the elements within the compound remains constant.

How was the Law of Constant Proportions discovered?

The Law of Constant Proportions was first articulated by the French chemist Joseph Louis Proust in the late 18th century. Through experimentation, Proust demonstrated that compounds maintain consistent proportions of their component elements, irrespective of the quantities of substances used or the conditions under which they reacted.

Why is the Law of Constant Proportions important?

This law is crucial because it provides insights into the fundamental nature of chemical compounds. Understanding that compounds maintain fixed proportions of their constituent elements allows chemists to predict and analyze reactions, ensuring reproducibility and reliability in scientific investigations and industrial processes.

Can you provide an example of the Law of Constant Proportions in action?

One classic example is water, composed of hydrogen and oxygen in a fixed ratio of 2:1 by mass. Whether obtained from a natural source or synthesized in a laboratory, water always consists of two parts hydrogen and one part oxygen. This fixed proportions illustrates the unchanging nature of compounds.

How does the Law of Constant Proportions apply in practical settings?

In industries such as pharmaceuticals, adherence to this law is crucial for ensuring the consistency and efficacy of medications. By precisely controlling the proportions of active ingredients, manufacturers can produce drugs with predictable effects, enhancing patient safety and therapeutic outcomes.

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