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State the law of constant proportions. Give one example to illustrate this law.

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Hint:The law of constant proportion was given by a French chemist named Joseph Proust in 1794. This law gives relation about the elements present in the chemical compound and their mass ratio.

Complete step by step answer:
The French Chemist named as Joseph Proust in 1794 gave the law of constant proportion by working on sulphides, metallic oxides and sulfates. The law of constant proportion was opposed by many scientists in 18 century but after the introduction of Dalton’s atomic theory, the law of constant proportion was highlighted and the relation between these two laws was put forward by a Swedish chemist named Jacob Berzelius in the year 1811.
The law of constant proportion states that the chemical compounds are formed by the elements which are present in the fixed ratio by mass. This shows that pure compounds will always contain the same elements which are present in the same ratio by mass.
The example showing the law of constant proportion is that the pure water is always composed of hydrogen and oxygen which are present in fixed ratio by mass that means a gram of water contains approximately 0.11 g of OH hydrogen and 0.88 g of oxygen which is present in the ratio of 1:8.

Note:
The law of constant proportion is also known as Proust’s Law or law of definite proportions. The law of constant proportion is not correct for every chemical compound and based on this there are many exceptions. Example: FeO which is an oxide of iron in which the ratio of iron and oxygen varies from 0.83:1 to 0.95:1.