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Law of Multiple Proportions states that when two elements combine with one another to form two or more compounds, the mass of one element that combines with an element of fixed mass is the ratio of whole numbers.
- The law was proposed by chemist John Dalton in 1803.
- It is also known as Dalton’s Law or Dalton’s Law of multiple proportions.
- Dalton mentioned about the law in his research work named “A New System Of Chemical Philosophy”.
- The Law of Multiple Proportions is among one of the Laws of Chemical Combinations.
- The Law of Chemical Combinations governs the transformation of matter from one form to another.
- It is based on compounds and their compositions.
- Compounds are chemical substances formed by combining two or more elements.
Read Also: Periodic Classification of Elements
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Key Terms: Law of Multiple Proportions, Law of Chemical Combinations, Compounds, Substances, Elements, Mass, Lavisoier’s law, Hydrogen, Oxygen
Law of Multiple Proportions Definitions
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Law of Multiple Proportions states that “If two elements combine to form more than one compound between them, the mass ratios of the second element which combine with a fixed mass of the first element will always be the ratios of small whole numbers.”
- The elements and the weights used in the process can be expressed as small whole numbers.
- The law is an important postulate of atomic structuring.
- It forms the basis of stoichiometry.
- The law of Multiple Proportions, Proust’s law of definite proportions and Lavisoier’s law of conservation mass are the framework of modern chemistry.
For Example: Hydrogen combines with oxygen to form two compounds, one water and another hydrogen peroxide.
The chemical reaction involved in the process is as follows: Hydrogen + Oxygen → Water
2 g + 16 g ----->18 g
Hydrogen + Oxygen → Hydrogen Peroxide
2 g + 32 g ------> 34 g
Example of Law of Multiple Proportions
In this reaction, the masses of Oxygen are 16g and 32g and Hydrogen’s fixed mass is 2g with a simple ratio of 16 : 32 or 1: 2 in both cases.
Also Read:
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| Periodic Properties of Element | Group 18 Elements | Clemmesen Reduction Reaction |
| Atomic mass of Element | Three States of Matter | Stephen Reaction Mechanism |
Examples of Law of Multiple Proportions
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Dalton explains the law of multiple proportions with an example of a carbon element and an oxygen atom. According to the law, when a carbon element is combined with an oxygen atom in various proportions, it forms two oxides.
- Let us consider a carbon element of a fixed mass of 100g, which can react with 266g of oxygen to form one oxide atom or with 133g of oxygen to form the other.
- The ratio of the oxygen masses that can react with carbon is given as 266:133 = 2:1, which is the ratio of small whole numbers.
Reaction of Carbon and Oxygen
Dalton has proposed that the two oxides have one oxygen atom and two oxygen atoms, respectively, for every carbon atom in his atomic theory. In modern notation, the first atom is CO (carbon monoxide), and the other is CO2 (carbon dioxide).
- Another example of the law of multiple proportions can be explained by comparing the ethane (C2H6) compound with the propane (C3H8) compound.
- Hydrogen’s weight combined with 1g of carbon is 0.252 g in the ethane and 0.224 g in the propane.
- The ratio is 1.125, which is equal to the ratio of two small numbers (9:8).
Limitations of Law of Multiple Proportions
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The limitations of law of Multiple Proportions are as follows:
- The Law of Multiple Proportions is valid till an element is present in one particular isotopic form in all its compounds.
- When an element exists in the form of different isotopes in its compounds, the Law of Multiple proportions does not hold well.
Read More: Anode and Cathode Important Notes
Law of Multiple Proportions: Dalton’s Atomic Theory
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Law of Multiple Proportions can be used to explain Dalton's Atomic Theory. Democritus first suggested the Atom's existence. However, it took almost two centuries to establish the Atom as a basic chemical entity, which was proposed by John Dalton.
Law of Multiple Proportions: Important Points of Dalton's Atomic Theory
The important points regarding Dalton atomic theory are as follows:
- All atoms of an element are identical in mass and properties.
- Atoms are indestructible and indivisible.
- A chemical reaction can represent the rearrangement of atoms.
- Compounds are produced by the combination of more than one variety of atoms.
Dalton's theory is valid in modern chemistry as well. It explains that atoms are destroyed by nuclear reactions but not by chemical reactions. A variety of atoms differ by their masses within an element called "isotopes", but the element's isotopes contain similar chemical properties.
Dalton’s Atomic Theory Video Explanation
Various chemical methods were quickly explained by Dalton in his theory and became the theoretical foundation in chemistry.
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Things to Remember
- According to the Law of Multiple Proportions, when two elements combine to form more than one compound, different masses of the same element will combine with similar masses.
- The law is popularly known as Dalton’s Law of multiple proportions.
- It also explains the Dalton atomic theory.
- The most common limitation includes that the law is valid until the element is present in one particular isotopic form.
- One of the examples of law includes the reaction between ethane and propane.
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Sample Questions
Ques: Consider a compound called carbon dioxide and carbon monoxide of 2.88 gm and 1.44 gm, respectively. Determine the mass ratio when it reacts with oxygen. (2 marks)
Ans. For carbon dioxide, 2.66g of oxygen/1.00g of carbon = 2.66. For carbon monoxide, 1.33 g of oxygen/1.00 g of carbon=1.33. The two ratios are in the proportion 2.66/1.33 =2:1.There the ratio of masses of oxygen that combine with the same mass of carbon is 2:1, i.e. a simple ratio.
Ques: Which law of chemical combination is obeyed by the above experimental data? Give its statement. (3 Marks)
Ans: The experimental data obey the Law of Multiple Proportions. It was proposed by Dalton in 1803. It states, "If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers."
- Considering the mass of dinitrogen as 28 g, masses of dioxygen combined will be 32, 64, 32, and 80 g in the given four oxides.
- These are in the ratio of 2: 4: 2: 5, which is a simple whole-number ratio.
- Hence, the given data obeys the law of multiple proportions.
Ques: Explain the law of multiple proportions with an example. (2 marks)
Ans: The law of multiple proportions states that "if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of another element, are in a ratio of small whole numbers. For Example, hydrogen and oxygen can combine to form water as well as hydrogen peroxide.
The law of multiple proportions
Here, the masses of oxygen (16g & 32g) are combined with a fixed mass of hydrogen (2g) to give a simple ratio i.e., 16:32 = 1:2
Ques: Give two examples of the Law of Multiple Proportions. (3 Marks)
Ans: Law of Multiple Proportions-
- A compound of Carbon and Oxygen: C and O combine to form two compounds, CO and CO2.
In CO2, 12g of C was combined with 16g of O.
In CO2, 12g of C was combined with 32g of O.
If the mass of C is fixed at 12 g, then the ratio of oxygen, which combines with the fixed mass of C (12g), is 16: 32 or 1: 2.
Thus, the mass of oxygen bears a simple ratio of 1 : 2 to each other.
- Compounds of Sulphate (S) and Oxygen (O): S forms two oxides with O, viz., SO2 and SO3
In SO2, 32g of S combines with 32g of O.
In SO3, 32g of S combines with 48g of O.
If the mass of S is fixed at 32g, then the ratio in the mass of oxygen, which combines with the fixed mass of S, is 32: 48 or 2: 3.
Thus, the mass of oxygen bears a simple ratio of 2: 3 to each other.
Ques: Which statements indicate that the law of multiple proportions is being followed?
(a) A sample of carbon dioxide taken from any source will always have carbon and oxygen in the ratio 1:2.
(b) Carbon forms two oxides, namely CO2 and CO, where masses of oxygen that combine with a fixed mass of carbon are in the simple ratio 2:1.
(c) When magnesium burns in oxygen, the amount of magnesium taken for the reaction is equal to the amount of magnesium in magnesium oxide formed.
(d) At constant temperature and pressure, 200 mL of hydrogen will combine with 100 mL of oxygen to produce 200 mL of water vapour. (4 Marks)
Ans: Statement (b) follows the Law of Multiple Proportion
Carbon combines with oxygen to form two compounds- carbon dioxide and carbon monoxide. In CO2 , 12 parts by mass of carbon combine with 32 parts by mass of oxygen, while in CO, 12 parts by mass of carbon combine with 16 parts by mass of oxygen.
Therefore, the masses of oxygen combined with a fixed mass of carbon (12 parts) in CO2 and CO are 32 and 16, respectively. These masses of oxygen bear a simple ratio of 32 : 16 or 2 : 1 to each other.
Ques: Give the Limitations of the Law of Multiple Proportions. (2 Marks)
Ans: The limitations of the Law of Multiple Proportions are as follows:
- The law of multiple proportions fails with the compounds of non-stoichiometric, Oligomers, and Polymers.
- It is valid until an element is present in one particular isotopic form in all its compounds. The Law of Multiple proportions does not hold well when an element exists in the form of different isotopes in its compounds.
Ques: How does this law of multiple proportions point to the existence of atoms? (2 Marks)
Ans: Law of multiple proportions shows that constituents combine in a definite proportion. These constituents may be atoms. Thus, the law of multiple proportions shows the existence of atoms that combine to form molecules.
Ques. What predictions are made by the Dalton Atomic Theory about a chemical. (2 Marks)
Ans. The predictions Dalton's Atomic Theory made about a chemical are as follows:
- Boiling Points
- Reactivity with other compounds
- Acidity
- Basicity
- Products obtained from chemical reaction
Ques. What is the Law of Conservation of Mass? (2 Marks)
Ans. The law of Conservation of Mass states that in all physical and chemical processes, the mass of reactants equals the mass of products. In this process, mass is neither created nor destroyed.
Ques. Explain (A) Atomic Mass
(B) Molecular Mass (2 Marks)
Ans. (A) Atomic Mass: Atomic Mass of a substance is defined as the average mass of electrons, protons and neutrons in an atom.
(B) Molecular Mass: The molecular Mass of a substance is defined as the sum of atomic masses of all the atoms present in a molecule of a substance.
Ques. What reactions are involved when hydrogen combines with oxygen. (2 Marks)
Ans. The reactions involved when hydrogen combines with oxygen are as follows:
Hydrogen + Oxygen → Water
Hydrogen + Oxygen → Hydrogen Peroxide
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