Law of Multiple Proportions: Definition, Examples & Limitations

Law of Multiple Proportions states that when two elements combine with one another to form two or more compounds, the mass of one element that combines with an element of fixed mass is the ratio of whole numbers. 

• The law was proposed by chemist John Dalton in 1803.
• It is also known as Dalton’s Law or Dalton’s Law of multiple proportions.
• Dalton mentioned about the law in his research work named “A New System Of Chemical Philosophy”.
• The Law of Multiple Proportions is among one of the Laws of Chemical Combinations.
• The Law of Chemical Combinations governs the transformation of matter from one form to another.
• It is based on compounds and their compositions.
• Compounds are chemical substances formed by combining two or more elements.​

Read Also: Periodic Classification of Elements

Key Terms: Law of Multiple Proportions, Law of Chemical Combinations, Compounds, Substances, Elements, Mass, Lavisoier’s law, Hydrogen, Oxygen

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Law of Multiple Proportions Definitions

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Law of Multiple Proportions states that “If two elements combine to form more than one compound between them, the mass ratios of the second element which combine with a fixed mass of the first element will always be the ratios of small whole numbers.”

• The elements and the weights used in the process can be expressed as small whole numbers.
• The law is an important postulate of atomic structuring.
• It forms the basis of stoichiometry.
• The law of Multiple Proportions, Proust’s law of definite proportions and Lavisoier’s law of conservation mass are the framework of modern chemistry.

For Example: Hydrogen combines with oxygen to form two compounds, one water and another hydrogen peroxide.

The chemical reaction involved in the process is as follows: Hydrogen + Oxygen → Water 

2 g + 16 g ----->18 g

Hydrogen + Oxygen → Hydrogen Peroxide

2 g + 32 g ------> 34 g

Example of Law of Multiple Proportions

In this reaction, the masses of Oxygen are 16g and 32g and Hydrogen’s fixed mass is 2g with a simple ratio of 16 : 32 or 1: 2 in both cases.

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Examples of Law of Multiple Proportions

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Dalton explains the law of multiple proportions with an example of a carbon element and an oxygen atom. According to the law, when a carbon element is combined with an oxygen atom in various proportions, it forms two oxides. 

• Let us consider a carbon element of a fixed mass of 100g, which can react with 266g of oxygen to form one oxide atom or with 133g of oxygen to form the other. 
• The ratio of the oxygen masses that can react with carbon is given as 266:133 = 2:1, which is the ratio of small whole numbers. 

Reaction of Carbon and Oxygen

Dalton has proposed that the two oxides have one oxygen atom and two oxygen atoms, respectively, for every carbon atom in his atomic theory. In modern notation, the first atom is CO (carbon monoxide), and the other is CO₂ (carbon dioxide).

• Another example of the law of multiple proportions can be explained by comparing the ethane (C₂H₆) compound with the propane (C₃H₈) compound.
• Hydrogen’s weight combined with 1g of carbon is 0.252 g in the ethane and 0.224 g in the propane.
• The ratio is 1.125, which is equal to the ratio of two small numbers (9:8).

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Limitations of Law of Multiple Proportions

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The limitations of law of Multiple Proportions are as follows:

• The Law of Multiple Proportions is valid till an element is present in one particular isotopic form in all its compounds. 
• When an element exists in the form of different isotopes in its compounds, the Law of Multiple proportions does not hold well.

Read More: Anode and Cathode Important Notes

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Law of Multiple Proportions: Dalton’s Atomic Theory

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Law of Multiple Proportions can be used to explain Dalton's Atomic Theory. Democritus first suggested the Atom's existence. However, it took almost two centuries to establish the Atom as a basic chemical entity, which was proposed by John Dalton. 

Law of Multiple Proportions: Important Points of Dalton's Atomic Theory

The important points regarding Dalton atomic theory are as follows:

• All atoms of an element are identical in mass and properties.
• Atoms are indestructible and indivisible.
• A chemical reaction can represent the rearrangement of atoms.
• Compounds are produced by the combination of more than one variety of atoms.

Dalton's theory is valid in modern chemistry as well. It explains that atoms are destroyed by nuclear reactions but not by chemical reactions. A variety of atoms differ by their masses within an element called "isotopes", but the element's isotopes contain similar chemical properties.

Dalton’s Atomic Theory Video Explanation

Various chemical methods were quickly explained by Dalton in his theory and became the theoretical foundation in chemistry.  

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Things to Remember

• According to the Law of Multiple Proportions, when two elements combine to form more than one compound, different masses of the same element will combine with similar masses.
• The law is popularly known as Dalton’s Law of multiple proportions.
• It also explains the Dalton atomic theory.
• The most common limitation includes that the law is valid until the element is present in one particular isotopic form.
• One of the examples of law includes the reaction between ethane and propane.

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