What Is the Law of Definite Proportions with Definition Formula and Examples
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 in Chemistry
1. What is the Law of Definite Proportions?
The Law of Definite Proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass, regardless of its source or method of preparation. This law, also called the Law of Constant Composition, means that:
- Every sample of a compound has identical mass percentages of its constituent elements.
- The composition does not change with quantity or origin.
- For example, water (H2O) always contains hydrogen and oxygen in a 1:8 mass ratio.
2. Who proposed the Law of Definite Proportions?
The Law of Definite Proportions was proposed by Joseph Louis Proust in 1799. He demonstrated through experiments that:
- Copper carbonate prepared in different ways had the same composition.
- Pure compounds always contain elements in fixed mass ratios.
- This law supported Dalton’s atomic theory.
3. What is an example of the Law of Definite Proportions?
A common example of the Law of Definite Proportions is water, where hydrogen and oxygen are always combined in a fixed 1:8 mass ratio. For example:
- In 18 g of H2O, 2 g is hydrogen and 16 g is oxygen.
- The formation reaction is: 2H2(g) + O2(g) → 2H2O(l).
- Any pure sample of water follows this same composition.
4. How is the Law of Definite Proportions different from the Law of Multiple Proportions?
The Law of Definite Proportions states that a compound has a fixed mass ratio of elements, while the Law of Multiple Proportions states that elements can combine in different simple whole-number ratios to form different compounds. For example:
- CO and CO2 both contain carbon and oxygen.
- In CO, the C:O mass ratio is 12:16.
- In CO2, the C:O mass ratio is 12:32.
5. Why is the Law of Definite Proportions important in chemistry?
The Law of Definite Proportions is important because it confirms that chemical compounds have fixed compositions based on atomic structure. Its importance includes:
- Supporting atomic theory.
- Helping determine empirical and molecular formulas.
- Forming the basis of stoichiometry calculations.
6. How do you verify the Law of Definite Proportions experimentally?
The Law of Definite Proportions can be verified by experimentally determining the mass ratio of elements in a pure compound and showing it remains constant. The steps are:
- Obtain a pure sample of a compound.
- Separate and measure the mass of each element.
- Calculate the mass ratio.
- Compare with another sample prepared differently; the ratio will be the same.
7. Does the Law of Definite Proportions apply to mixtures?
The Law of Definite Proportions does not apply to mixtures because mixtures do not have fixed compositions. In mixtures:
- Components can be present in any proportion.
- Composition can vary from sample to sample.
- For example, air contains variable percentages of nitrogen and oxygen.
8. How does the Law of Definite Proportions relate to chemical formulas?
The Law of Definite Proportions explains why chemical formulas represent fixed ratios of atoms in a compound. For example:
- NaCl always contains sodium and chlorine in a 1:1 atomic ratio.
- This corresponds to a fixed mass ratio of 23:35.5.
- The formula reflects the constant composition of the compound.
9. Are there any exceptions to the Law of Definite Proportions?
The Law of Definite Proportions generally applies to pure compounds, but some non-stoichiometric compounds show slight deviations. These include:
- Metal oxides like FeO, which may have variable composition.
- Defects in crystal structures causing small ratio changes.
- Such cases are exceptions and mostly occur in solid-state chemistry.
10. How do you calculate the mass ratio in the Law of Definite Proportions?
To calculate the mass ratio in the Law of Definite Proportions, divide the mass of each element in the compound and express it in simplest form. For example, in CO2:
- Atomic mass of C = 12 g/mol.
- Atomic mass of O2 part = 2 × 16 = 32 g/mol.
- Mass ratio C:O = 12:32 = 3:8.
