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.
Polymers
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.
FAQs on Law of Definite Proportions
1. What is the Law of Definite Proportions?
The Law of Definite Proportions, also known as Proust's Law or the Law of Constant Composition, states that a given chemical compound always contains its component elements in a fixed ratio by mass, regardless of its source or method of preparation. For example, any sample of pure water (H₂O) will always consist of approximately 88.8% oxygen and 11.2% hydrogen by mass.
2. Who proposed the Law of Definite Proportions?
The Law of Definite Proportions was proposed by the French chemist Joseph Proust. He formulated this law based on his experiments conducted between 1798 and 1804, where he observed that compounds like copper carbonate always contained the same proportions of elements by mass.
3. What are some common examples of the Law of Definite Proportions?
Common examples that illustrate this law include:
- Water (H₂O): It is always formed by combining hydrogen and oxygen in a fixed mass ratio of 2:16, which simplifies to 1:8. This means 1 gram of hydrogen combines with 8 grams of oxygen to form 9 grams of water.
- Carbon Dioxide (CO₂): It is always formed from carbon and oxygen in a fixed mass ratio of 12:32, which simplifies to 3:8. Any pure sample of CO₂ will adhere to this ratio.
4. How is the Law of Definite Proportions applied in chemistry?
This law is a fundamental concept in chemistry, particularly in stoichiometry. It helps establish the purity of a compound and is essential for writing correct chemical formulas. For example, knowing that a compound has a fixed composition allows chemists to calculate the amount of reactants needed and the products that will be formed in a chemical reaction, as per the CBSE 2025-26 syllabus.
5. How did John Dalton's theory support the Law of Definite Proportions?
While Joseph Proust proposed the law based on experimental observations, John Dalton's atomic theory provided the theoretical explanation for it. Dalton postulated that elements consist of atoms, and compounds are formed when atoms of different elements combine in simple, whole-number ratios. Since atoms of an element have a fixed mass, a compound made of a fixed ratio of atoms will naturally have a fixed ratio of masses, thus validating the Law of Definite Proportions.
6. What is the difference between the Law of Definite Proportions and the Law of Multiple Proportions?
The key difference lies in the number of compounds they describe:
- The Law of Definite Proportions applies to a single compound. It states that a specific compound (like H₂O) always has the same fixed ratio of elements by mass.
- The Law of Multiple Proportions applies when two elements form more than one compound (like CO and CO₂). It states that the masses of one element that combine with a fixed mass of the second element are in a ratio of small whole numbers.
7. Does the Law of Definite Proportions have a formula?
No, the Law of Definite Proportions is not a mathematical formula but a fundamental principle or law in chemistry. It describes the consistent nature of a compound's composition. While we use mathematical ratios (like 1:8 for water) to express this law, there is not a single universal algebraic equation for it. It is a concept demonstrated through calculation for each specific compound.
8. Why is the Law of Definite Proportions not universally true?
The Law of Definite Proportions is not universally applicable due to a few key exceptions recognised in modern chemistry. These include:
- Non-stoichiometric compounds (Berthollides): These compounds do not have a fixed composition. For example, wüstite (an iron oxide) can have a formula ranging from Fe₀.₈₃O to Fe₀.₉₅O.
- Isotopes: Elements can exist as isotopes with different masses. The mass ratio in a compound can vary slightly depending on the isotopic source of its elements.
- Polymers: Many natural and synthetic polymers can have varying compositions and molecular weights, even within a sample considered "pure".
9. If you mix hydrogen and oxygen in a 1:1 mass ratio, do you get water? Why or why not?
No, you would not get pure water. You would get a mixture containing water and leftover, unreacted hydrogen. According to the Law of Definite Proportions, water (H₂O) must form in a fixed mass ratio of hydrogen to oxygen of 1:8. If you mix them in a 1:1 mass ratio, 1 gram of oxygen will react with only 0.125 grams of hydrogen, leaving the rest of the hydrogen unreacted.
