What is the Law of Reciprocal Proportions?, Important Points, JEE 2025
Shrivastav 6 Jun, 2024
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Law of Reciprocal Proportions
Chemistry is a fascinating world filled with laws and principles that govern the behaviour of matter. One such essential principle is the Law of Reciprocal Proportions, which provides insights into the relationship between different elements in compounds. Let's embark on a journey to unravel this law in simple terms and explore its significance for students studying chemistry.
What is the Law of Reciprocal Proportions?
The Law of Reciprocal Proportions is a fundamental principle in chemistry that describes the relationship between the masses of different elements that combine to form compounds. It states that when two elements combine with a fixed mass of a third element to form two or more different compounds, the ratios of the masses of the first two elements to each other are simple whole-number ratios.
Law of Reciprocal Proportions Important Points
To grasp this law better, let's break it down into simpler terms. Imagine you have two elements, Element A and Element B, which can combine with a third element, Element C, to form different compounds. According to the Law of Reciprocal Proportions, if Element A combines with Element C to form one compound and Element B combines with Element C to form another compound, the ratios of the masses of Element A to Element B will be simple whole numbers.
Law of Reciprocal Proportions In Other words
When three elements combine with each other in combination of two and form three compounds then the ratio of masses of two elements combining with fixed mass of the third and the ratio in which they combine each other bear a simple whole number ratio to each other. This law was given by Richter in 1792.
This law can be understood easily with the help of the following examples.
⇒ Let us consider three elements – hydrogen, sulphur and oxygen. Hydrogen combines with oxygen to form H 2 O whereas sulphur combines with it to form SO 2 . Hydrogen and sulphur can also combine together to form H 2 S. The formation of these compounds is shown in fig.
In H 2 O, the ratio of masses of H and O is 2:16.
In SO 2 , the ratio of masses of S and O is 32 :32.
Therefore, the ratio of masses of H and S which combines with a fixed mass of oxygen (say 32 parts) will be
4:32 i.e. 1:8 ………(i)
When H and S combine together, they form H 2 S in which the ratio of masses of H and S is:
2:32 i.e., 1 : 16 ……….(ii)
The two ratios (i) and (ii) are related to each other as:
i.e., they are whole number multiples of each other. Thus, the ratio masses of H and S which combines with a fixed mass of oxygen is a whole number multiple of the ratio in which H and S combine together.
Solved Example Of Law of Reciprocal Proportions
Q.1 : Methane contains 75% carbon and 25% hydrogen, by mass. Carbon dioxide contians 27.27% carbon and 72.73% oxygen, by mass. Water contains 11.11% hydrogen and 88.89% oxygen, by mass. Show that the data illustrates the law of reciprocal proportion.
Sol. Methane and carbon dioxide, both contains carbon and hence, carbon may be considered as the third elements. Now, let the fixed mass of carbon = 1 gm. Then, the mass of hydrogen combined with 1 gm carbon in methane
and the mass of oxygen combined with 1 gm carbon in carbon dioxide
Hence, the mass ratio of hydrogen and oxygen combined with the fixed mass of carbon,
As r 1 and r 2 are same, the data is according to the law of reciprocal proportion.
Law of Reciprocal Proportions FAQs
Q.1 : What is the Law of Reciprocal Proportions?
Q.2 : How does the Law of Reciprocal Proportions apply to real-life examples?
Q.3 : Why is the Law of Reciprocal Proportions important in chemistry?
Q.4 : How does the Law of Reciprocal Proportions support the atomic theory?
Q.5 : Can you explain the Law of Reciprocal Proportions in simpler terms?
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