Question

Common salt obtained from Clifton beach contained \[60.75\% \] chlorine while
\[6.40\,g\] of a sample of common salt from Khewra mine contained \[3.888g\] of chlorine. State the law illustrated by these chemical combinations.
A. Law of reciprocal proportion
B. Law of multiple proportion
C. Law of constant composition
D. None of the above

Answer 1

Hint: This law states that “a chemical compound is always found to be made up of the same elements combined together in the same proportions by weights”. This law is also called law of definite proportion. This law was firstly started by Joseph Proust and that is why it is also called Proust’s law.

Complete step by step answer:
Case I:
Common salt from Clifton beach contains \[ = 60.75\% \,C{l_2}\]​
\[100\,g\] of common salt from Clifton beach \[ = 60.75\% \,C{l_2}\]​
\[1\,g\] of salt of common salt from Clifton beach \[ = \dfrac{{60.75}}{{100}} = 0.6075\,g\]
\[\Rightarrow 0.6075g\,\,of\,C{l_2}\]

Case II:
\[6.40\,g\]of \[NaCl\] from Khewra mine\[ = 3.888g\,\,of\,C{l_2}\]​
\[1\,g\] of \[NaCl\] from Khewra mine \[ = \dfrac{{3.888}}{{6.40}} = 0.6075\,g\]
\[\Rightarrow = 0.6075g\,\,of\,C{l_2}\]
Thus, the weight of \[C{l_2}\]​ in \[1\,g\] of salt in both the cases is the same. Hence, the law of constant composition is verified.

The correct option is (C) i.e., Law of constant composition.

Note: you should also know the other mentioned law in the given option. It would be helpful to polish your knowledge. Let’s start with the law of reciprocal proportions.
The law of reciprocal proportions states that if two different elements combine separately with the same weight of a third element, the ratio of the masses in which they do so are either the same or a simple multiple of the mass ratio in which they combine."
Law of Multiple proportion states that when two elements combine to form more than one compound, the mass of one element, which combines with a fixed mass of the other element, will always be ratios of whole numbers.
Example:
\[CO\, = \,12gm\,\]of carbon \[ + 16\,gram\]of Oxygen.
\[CO{\,_2} = \,12gm\,\]carbon\[ + 32\,gram\] of Oxygen .
Thus ratio of mass of oxygen in the given two compounds is \[16:32\, = \,1:2\]