Is the law of constant composition true for all types of compounds? Explain why or why not?

Hint: The law of constant composition states that “the proportions of the elements in a compound are always the same, no matter how the compound is made.” This law is also known as the law of constant proportion. There are two other laws for the chemical composition, law of multiple proportion and law of equivalent proportion.

Complete answer:
The law of constant composition is valid for a large number of compounds but the law is not true for the following compounds:
1. Non-stoichiometric compounds.
2. Compounds made up of isotopes of different elements.
The law is true for the compounds obtained from one isotope, like ${}^{12}C/{}^{14}C$.
Hence, no, the law of constant composition is true for all types of compounds.

Note:
Elements containing a mixture of stable isotopes also form non-stoichiometric compounds. Some of the elemental positions in the crystal structure are replaced by their isotopes, which makes a difference in the actual composition of the crystal.