Question

Why does the law of definite proportion does not hold true when the compound is obtained by using different isotopes of the combining elements ?

Answer 1

It is because, the law of definite proportion states that a chemical compound have the mass of constituent elements in definite ratio.
But, when the same compound is taken for two different isotopes of same elements, the mass of the isotopes changes and so the ratio of mass will also change and so the rule does not holds to be true in such cases.