Popular Branches
MBA
B.Tech
BBA
BSc
Updated on 15th February, 2023 , 5 min read
The law of constant proportion states that the elements present in a chemical compound are in a fixed ratio by mass. This holds true for every pure sample of the compound, regardless of its origin. For instance, pure water will always have hydrogen and oxygen in the same proportion by mass, with approximately 0.11 grams of hydrogen and 0.88 grams of oxygen in a 1:8 ratio for every gram of water.
The law of constant proportion, also known as Proust's law or the law of definite proportion, states that the mass ratio of elements in a compound is always constant. The illustration below shows the mass ratio of various compounds, with the number of atoms in each element indicated beneath the mass ratio. For instance, the ratio of nitrogen to oxygen atoms in a nitrogen dioxide (NO2) molecule is 1:2, while the corresponding mass ratio is 14:32 (or 7:16).
The law was first formulated by French scientist Joseph Proust in 1794, based on his studies of sulphides, metallic oxides, and sulfates. Despite initial resistance in the scientific community, the law gained acceptance with the advent of Dalton's atomic theory and its validation by Swedish chemist Jacob Berzelius in 1811.
Let us consider an unknown substance, "X." We perform tests to identify the elements present and find that it contains hydrogen and oxygen in their pure forms. Our knowledge suggests that this combination can produce two stable compounds:
To differentiate between the two, we break down "X" and weigh each component separately. If the weight percentages match those of water in the table, we can conclude that "X" is water.
Let's pause here to examine the basis for the Law of Constant Proportion. We are all familiar with the Law of Conservation of Mass, which states that mass cannot be created or destroyed. This law provides the foundation for constant proportion. For example, consider a 1-mole water sample in a sealed system. If we subject it to electrolysis, the water will vaporize into hydrogen and oxygen molecules. Upon separating and weighing the two, we can demonstrate the constant proportion of hydrogen to oxygen in the water.
Here are a few examples of the Law of Constant Proportion:
These examples demonstrate that the Law of Constant Proportion applies to all chemical compounds and that the ratio of elements in a compound is always constant, regardless of the source or the conditions under which it is formed.
The Law of Constant Proportion can be stated in a simple formula as follows:
"In a given chemical compound, the elements are always present in the same proportion by mass."
This means that the ratio of the masses of the elements in a compound is always constant, regardless of the source or the conditions under which it is formed. For example, in water (H2O), the mass ratio of hydrogen to oxygen is always 1:8, regardless of where or how the water is obtained. This principle is a fundamental concept in chemistry and is essential for understanding the behavior and properties of chemical compounds.
The law of constant proportion is an important principle in chemistry for several reasons:
Overall, the law of constant proportion is a fundamental principle in chemistry that provides a basis for understanding chemical compounds and their behavior.
Here are the Exceptions to the Law of Constant Proportion explained in points:
Ans: The law of constant proportion asserts that the elements in a pure chemical compound are consistently present in fixed mass ratios.
Ans: The Law of definite proportion asserts that elements in a particular compound are combined in a constant mass ratio.
Ans: No, it's False. The law of constant proportion or Law of definite proportion states that:
In a chemical substance, the elements are always present in a definite proportion by mass.
Let's take an example of H2O.
Here, the chemical substance is H2O, the elements are H and O and the ratio will be 1:8 by mass.
Ans: According to the constant proportion theorem, the elements in a chemical reaction are always present in a definite proportion by their mass.
Pilot Salary in India 2024: Starting Salary, Requirements, Qualifications, Per Month Salary
By - Nikita Parmar 2024-09-06 10:59:22 , 6 min readThe law of definite proportions, also known as the law of constant proportions, states that the individual elements that constitute a chemical compound are always present in a fixed ratio (in terms of their mass). This ratio does not depend on the source of the chemical compound or the method through which it was prepared.
The ratio of elements in non-stoichiometric compounds varies from sample to sample. Therefore, these compounds are an exception to the law of constant proportions. Samples of elements that vary in their isotopic composition can also defy the law of definite proportions since the masses of two different isotopes of an element are different. Natural polymers are also known to disobey the law of constant proportions.
Even though this law is easily understandable today, it was of great use in the late 18th century when chemical compounds did not have any proper definition. The law of definite proportions also contributed to the development of Dalton’s atomic theory.
An example showing the law of constant proportion is that pure water is always composed of hydrogen and oxygen which are present in a fixed ratio by mass which means a gram of water contains approximately 0.11 g of OH hydrogen and 0.88 g of oxygen which is present in the ratio of 1:8.
The French scientist J. L. Proust proposed the law of constant proportions.