Law of Definite Proportions

Dhristi JEE 2022-24

What is the Law of Definite Proportions?

Law of Definite Proportion, also called Proust's Law or Law of Constant Composition, defines that the elements that make up a chemical compound are usually arranged in a specified mass ratio regardless of the source or preparation. The law of definite proportion can also be expressed in another way.

How does the Law of Definite Proportions work?

Using the law of definite proportions, the composition of compounds will always be the same by mass. In Chemistry, stoichiometry is based on this law.

Statement: Chemical compounds consist of elements that are always present at fixed ratios (in terms of their mass) according to the law of definite proportions as well as the law of constant proportions. In this ratio, neither the source nor the method of preparation of the chemical compound is relevant.

Explanation: Chemical compounds, according to the law of constant proportions, are made of elements present in a fixed ratio by mass. The concentration of each element in a compound will always be the same by mass regardless of the source of the sample.

Nitrogen and oxygen atoms are always in a 1:2 ratio in the nitrogen dioxide molecule (NO2). Consequently, Nitrogen has the same structure as oxygen.

A Brief History of the Law of Definite Proportions

Proust’s Law

In the period between 1798-1804, French chemist Joseph Proust experimented with copper carbonate and water to develop a law of definite composition or proportions. Proust formulated his observations in what is now known as Proust's Law in 1806. As determined by mass, chemical compounds are composed of constant and defined ratios of elements. As an example, carbon dioxide consists of one carbon atom and two oxygen atoms. As a result, carbon dioxide can be described by the fixed ratio of 12 (mass of carbon):32 (mass of oxygen), or simplified as 3:8.

Disagreements with the Proust’s Law

Several chemists disagreed with Proust's theory at the time, particularly another French chemist, Claude Louis Berthollet. The French scientist believed that elements could mix in any ratio. A chemist called John Dalton proposed that chemical compounds were composed of atoms belonging to different elements. This idea was supported at an atomic level, however, as Dalton proposed that chemical compounds were composed of set formulas of atoms. In Dalton's law of multiple proportions, elements can combine to yield new combinations of elements in a compound. In such a scenario, the ratio of the elements within a compound can be expressed as a whole number, which is an extension of Dalton's law of definite composition.

Non-Stoichiometric Compounds/Isotopes

The Law of definite proportions is not true universally, despite its considerable usefulness in modern chemistry. Different samples of a compound may have different elemental compositions due to non-stoichiometry. Compounds like these are subject to the Law of Multiple Proportions. As an example, the iron oxide wüstite, which may contain anywhere between 23 and 25 oxygen atoms by mass, holds 0.83 to 0.95 iron atoms for each oxygen atom. It is given as FeO, but the crystallographic vacancies result in FeO.95O being the ideal formula. The measurements of Proust were generally not accurate enough to detect these differences.

Furthermore, the composition of the element can differ depending on its source; therefore, the mass of the element can differ even within a pure, stoichiometric compound. Due to processing in the atmosphere, astronomy, crust, oceans, and deep Earth that tend to concentrate few environmental isotopes, one can use this variation in radiometric dating. Except for hydrogen and its isotopes, most of the time, the effect is small, but the instrumentation of today allows us to measure it.


Additionally, the composition of several natural polymers differs, even when they are considered "pure". As a rule, polymer molecules are not considered pure chemical compounds except when their molecular weights are uniform (which is mono distribution), and their stoichiometry is constant. They still might be in violation of the Law in these rare cases due to the isotopic variations.

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FAQs (Frequently Asked Questions)

1. What are the laws of Multiple Proportions?

Usually, the Law of multiple proportions can be paired with the Law of definite proportions. This Law states, in order for two compounds to be similar, they must contain identical elements which are bonded in identical ratios. In other words, water will always be made from one oxygen atom and two hydrogen atoms. If we combine both oxygen and hydrogen in any other ratio, the compound cannot become water.

2. Explain the Law of Constant Proportion?

"A chemical compound always holds the similar elements which are combined together in a similar proportion by mass," which is said by a chemist, Joseph Proust. Therefore, a molecule of salt in the cellar will hold exactly the similar proportion of chlorine and sodium (that is, with the ratio 1:1) as any other salt molecule universally.

3. What is the Law of Variable Proportion?

It is essential to economic theory to understand the Law of Variable Proportions. By examining a one-factor variable, it focuses on the production function, while leaving the quantities of other factors unchanged. As opposed to this, it refers to the input-output relation that is used when one input quantity is differentiated in order to improve the output.

4. Explain the Law of Demand?

According to the Law of Demand, the quantity bought differs inversely with price. Generally speaking, the more expensive an item is, the lower its demand. The reason for this is that marginal utility diminishes with higher prices. Consumers use the first units of any economic good they purchase to address the most pressing needs, then use the subsequent units of that good for decreasingly important purposes.

5. Application of the Law of Definite Composition

As ionic compounds require certain ratios to achieve electrical neutrality, the law of definite composition also applies to molecular compounds with a fixed composition. The law of definite composition does not apply in all cases. Examples include ferrous oxide, which is a compound that is not stoichiometric. Furthermore, isotopic mixtures are not taken into account by the law of definite composition.

6. The Law of Definite Proportions and its Exceptions

However, the law of definite proportions is not universally applicable in chemistry. A compound's elemental composition can vary from sample to sample if it is non-stoichiometric in nature. Wustite, for example, is a type of iron oxide that has an elemental composition varying between 0.83 and 0.95 iron atoms for every oxygen atom (23%–25% oxygen by mass). There is a variation in the crystal structure of iron oxide, which means the ideal formula is FeO. Wustite's formula is Fe0.95O.

It also varies according to the source of an element's isotopic composition. A stoichiometric compound with the same formula will have a different mass depending on its origin.

Despite not being truly chemical compounds in the strictest sense, polymers differ in their element composition by mass.