Question

What are the limitations of the law of definite proportions?
(A) The law is not true if different isotopes of elements are involved in making a chemical compound.
(B) The law is not true if elements combine in the same ratio but produce different compounds.
(C) Both A and B.
(D) None of the above.

Answer 1

Hint: If we know about the law of definite proportion, it will be easy for us to determine the limitation of the law. Law of definite proportion is also called the law of constant composition. Dalton used the law of definite proportion for deriving atomic model theory.

Complete answer:
Before moving to the limitation of law of definite proportion, we should know about the law of definite proportion. The law of definite proportion is also called the Proust’s law. According to this law of definite proportion the elements in a chemical compound will always contain the same proportion by mass (or in fixed ratio). We can further understand this law by using an example of water. When we consider a pure sample of water, it contains 88.81% of Oxygen and 11.19% of hydrogen by mass.
Now let us see what are the limitations of law of definite proportions. The above given law is not applicable if the two elements combine in the same ratio and form different compounds. This law is also not true, if the elements with different isotopes react to give a chemical compound.

Therefore, the correct answer is option (C) Both A and B.

Additional information:
Let us see what law of conservation of mass is. Law of conservation of mass states that Mass can neither be created nor be destroyed. It can be transformed from one form to another.

Note: Dalton, along with the other laws, he used the law of definite proportion for deriving the atomic model theory. The other law which is used by Dalton to derive atomic theory is the Law of conservation of mass.