Question

Why is law of definite proportions important?

Answer 1

During the conversion of one substance into another via chemical reaction some basic rules are made which are known as laws of chemical combination. Law of definite proportion or law of constant composition is a type of law of chemical combination.

Complete answer:
The law of constant composition basically deals with the composition of different elements present in a particular compound. This law was first described by scientist Joseph Proust. According to the law of constant proportion “a pure compound will always contain the same elements which combine with each other in a definite proportion by weight”.
According to this law, the proportion of an element will always remain the same in a compound irrespective of its source.
For example: there are number of methods to generate carbon dioxide gas such as
By burning of candle or waste material at house, the chemical reaction for formation of carbon dioxide will be expressed as:
$C + {O_2} \to C{O_2}$
When we heat limestone, it also generates carbon dioxide gas which is expressed as:
$CaC{O_3}\underrightarrow \Delta CaO + C{O_2}$
During the acid rain when hydrochloric acid in air react with marbles of monuments which are made up of calcium carbonate will generate carbon dioxide which is expressed as:
$CaC{O_3}$ $+ 2HCl \to CaC{l_2} + C{O_2} + {H_2}O$
From the above examples we see that carbon dioxide is made up of one carbon and two oxygen atoms.
Molecular mass of carbon dioxide is $44$ g.
In one molecule of $C{O_2}$ , the total mass of carbon present is $12$ g.
When we calculate the proportion of carbon in carbon dioxide molecule is $= \dfrac{{12}}{{44}}$ g
In one molecule of $C{O_2}$ , the total mass of oxygen present is $32$ g.
When we calculate the proportion of oxygen in a carbon dioxide molecule is $= \dfrac{{32}}{{44}}$ g.
It shows that for every molecule of carbon dioxide, $12$ g of carbon and $32$ g of oxygen is present. So, the ratio of presence of carbon and oxygen in carbon dioxide will become-
One molecule of $C{O_2}$ $= \dfrac{{{M_{\left( C \right)}}}}{{{M_{\left( O \right)}}}}$
One molecule of $C{O_2}$ $= \dfrac{{12}}{{32}}$
After solving the above equation, we get
One molecule of $C{O_2}$ $= \dfrac{3}{8}$
Hence, we can conclude that for every molecule of carbon dioxide irrespective of its source of origin the proportion of carbon and oxygen in it is $3:8$ .
$\Rightarrow$ Law of definite proportions is important because it gives the detail about the proportion of elements in a compound by weight irrespective of its source.

Note:
There are some limitations of the law of definite proportion like it does not explain the proportion of elements which are formed by different isotopes. This law does not explain the formation of non-stoichiometric compounds like iron oxide.