Table of Content –
- What is law of definite proportions / constant composition ?
- Importance of law of definite proportions
- Limitations of law of definite proportions
- Problems based on law of definite proportions/ constant composition
- Frequently asked Questions
What is law of definite proportions / constant composition ?
The law of definite proportions or constant composition states that –
“All pure samples of a particular chemical compound, the constituent elements are always combined in the same proportions by mass, regardless of the origin or mode of preparation of the compound”.
This is the second law of chemical combination supported by the atomic theory. It was proposed by Proust. It establishes the nature of chemical compounds. This law means when elements react chemically, they combine in specific proportions by mass, not in random proportions. This law forms the basis for the definition of a chemical compound.
Importance of Law of definite proportions –
The law of definite proportions means that a specific ratio of elements found in a chemical compound. If the ratio of elements is different from that specific ratio then it is not the same compound and therefor has a different name.
Limitations of law of definite proportions –
- The law fails to be applied in the case of isotopic elements. Isotopes are atoms of the same element which have the same atomic number but different atomic weights. For example, chlorine has two stable isotopes, chlorine-35, and chlorine-37.
- This is also not applicable when there are possibilities of making two or more compounds from the same elements. For example – ethanol and dimethyl ether.
As both the compounds have the same chemical formula and the elements carbon, hydrogen, and oxygen are present in the same ratio, that is, 24:6:16 or 12:3:8 in mass. But the same ratio is given in two different compounds, which are ethyl alcohol and dimethyl ether.
Example – 1
A sample of pure water, whatever the source, always contains 88.9% by mass of oxygen and 11.1% by mass of hydrogen. A water molecule (H2O) consists of two hydrogen atoms each of relative mass of 1 and one oxygen atom of relative mass of 16 (rounded to nearest integer number). By putting a sensible unit measurement this means that there are 2 g of hydrogen and 16 g of oxygen in a sample of 18 g of water. The ratio is 1 : 8. Thus, a sample of, say, 51.435 g of water always contain (51.435 x 1/9) or 5.715 g of hydrogen and (51.435 x 8/9) or 45.720 g of oxygen. The ratio, again, is 1 hydrogen to 8 oxygen. This rule applies for water found anywhere in the universe obtained from river, sea, rain, coconut, etc. and the mass proportion is always the same for any given unit measurement (kilogram, pound etc.).
Example – 2
All pure sample of carbon dioxide (CO2) contain carbon and oxygen combined in ratio 1: 2 irrespective of whether the carbon dioxide is exhaled during respiration, formed from the combustion of organic compounds, a product of fermentation, etc.
Example – 3
The compound cupric oxide may be prepared by any one of the following methods –
• Heating copper in oxygen.
• Dissolving copper in nitric acid and igniting the cupric nitrate formed.
In both the cases, the ratio of copper : oxygen by mass is always constant.
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Frequently asked Questions –
- Name the scientist who gave the law of constant proportions.
Ans – Joseph Proust
- In a compound, the constituents are present in definite proportion by ___________________. Ans – mass / weight
- A sample of pure water, whatever the source, always contains _______- by mass of oxygen and 11.1% by mass of hydrogen.
Ans – 88.9%
- In , the proportion of H and O by mass is –
Ans – 1:16
- State an experiment to prove the law of constant proportions.
Ans – If 100 g of a sample of pure water, whatever the source is decomposed by passing electricity, it always contains 88.9% by mass of oxygen and 11.1% by mass of hydrogen.
- State the law of constant proportions.
- Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 gm of hydrogen gas ?
Ans: Since hydrogen and oxygen combine in the ratio of 1:8 by mass, it means that x gm of hydrogen and 8x gm of oxygen will be required to form water.
So, Oxygen required to react with 3 gm of hydrogen to form water = 3 x 8 = 24 gm.
- Does the Law of Definite Proportions apply to Mixtures and the Elements?
Ans – It does not apply to the elements and the mixtures because an element has only one atom. The mixture does not have fixed composition like the compound.