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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.
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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