Explain the law of constant proportion with example.


The law of constant proportion is also known as the law of definite proportion. This law, which was introduced by Proust, stated that in a compound, the elements are always present in definite proportions by mass.

For example, pure water obtained from any source (well, river, lake, or sea) and from any country (India, Russia, America, etc.) will always contain two hydrogen atoms and one oxygen atom. Hydrogen and oxygen respectively combine together in the ratio of 1:8 by mass to form water. The ratio by the number of atoms for water will always be H: O = 2: 1. Thus, 18 g of water contains 2 g of hydrogen and 16 g of oxygen, or the ratio of masses of hydrogen to oxygen will always be 1:8.

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