Question

Potassium combines with two isotopes of chlorine ($^{35}Cl$ and $^{37}Cl$) respectively to form two samples of KCl. Their formation follows the law of:
(A) constant proportions
(B) multiple proportions
(C) reciprocal proportions
(D) none of these

Answer 1

Hint: Write down the masses of potassium and the two isotopes of chlorine. Then find the mass of two samples of potassium chloride. If their masses are equal then their formation follows a definite proportion. However, if the masses vary although the compound is the same then we can say that the compound can exist in multiple proportions.

Complete step by step answer:
The law of constant proportion states that chemical compounds are made up of elements and their masses are in a fixed ratio. This implies that any pure sample of a compound, irrespective of the source will always consist of the same elements that are present in fixed ratio of mass.
The law of multiple proportion states that when two elements combine with each other to form greater than one type of compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.
The law of reciprocal proportions also referred to as law of equivalent proportions is one of the basic laws under stoichiometry. It relates the proportions in which elements combine across a number of other elements.
$^{35}Cl$ and $^{37}Cl$ combine with potassium to form KCl however their molecular mass are not equal. However, the ratio of masses of the two isotopes is constant. Hence their formation follows the law of multiple proportions.
So, the correct answer is “Option B”.

Note: The law of conservation of mass is an important law while determining mass of unknown products like gases in a chemical reaction. The law states that no mass is gained or lost in a chemical reaction. This implies that the total mass of reactants is equal to the total mass of products.