Law of definite proportion is not true for all types of compounds. The law is not valid for non-stoichiometric compounds.These are compounds whose compositions vary from sample to sample.Mainly inorganic compounds come into this class.An example of these types of compound is wustite, which has the formula FeO, but in actual the stoichiometry in this compound is Fe0.95O. This is so because some of the Fe^+2 gets easily oxidised to Fe^+3 and takes place of Fe^+2 in the lattice. To balance the charge 3 Fe^+2 atoms are being replaced by 2 Fe^+3 atoms. Elements having mixture of stable isotopes also form non-stoichiometric compound. Some of the element position in the crystal structure gets replaced by their isotopes and causes difference in actual composition of the crystal. For example , natural carbon, silicon, and germanium show this type of defect.

The law of constant composition states that a given compound will always have the same ratio of elements by mass. In other words, the relative proportions of the elements in a compound are fixed and independent of the source or method of preparation of the compound. However, this law does not hold true for non-stoichiometric compounds.

**Non-stoichiometric Compounds:**
Non-stoichiometric compounds are compounds that do not have a fixed and definite ratio of elements. This means that the relative proportions of the elements in these compounds can vary. Non-stoichiometric compounds are often formed due to the presence of defects in the crystal lattice structure of the compound.

**Defects in Crystal Lattice:**
In a crystal lattice, the atoms or ions are arranged in a regular and repeating pattern. However, in non-stoichiometric compounds, there can be defects in this regular pattern, such as vacancies or interstitial atoms. These defects can lead to a variation in the ratio of elements in the compound.

**Example:**
One example of a non-stoichiometric compound is iron(II) oxide, FeO, which can exist in two forms: Fe0.95O and Fe0.94O. In these compounds, the ratio of iron to oxygen is not fixed and can vary slightly. This variation is due to the presence of defects in the crystal lattice structure of the compound.

**Implications of Non-Stoichiometric Compounds:**
The presence of non-stoichiometric compounds can have important implications in various fields. For example, in materials science, non-stoichiometric compounds can exhibit unique properties that make them useful in specific applications. These compounds can also impact the stoichiometry of chemical reactions involving them, as the variation in the ratio of elements can affect the reaction kinetics and thermodynamics.

**Conclusion:**
In conclusion, the law of constant composition does not hold true for non-stoichiometric compounds. These compounds have a variable ratio of elements due to defects in their crystal lattice structure. The presence of non-stoichiometric compounds can have significant implications in various fields and can impact the stoichiometry of chemical reactions.

Law of constant composition :::::elements always combine in a fixed ratio by mass to form stable compounds.(e. g. H and O always combine in ratio 2/16 to form H2O)H2 + 1/2(O2)===>H2O
___________ this ratio always remain fixed irrespective of source or method of synthesis of the compounds ,,,,,,,,,,,,,,,,,combination ratio by mass will be same_______