CBSE Class 11-science Answered

Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry) is the calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions (chemicals).

The law of definite proportions and also the elements states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition. For example, oxygen makes up ⁸/₉ of the mass of any sample of pure water, while hydrogen makes up the remaining ¹/₉ of the mass. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry.

Non-Stoichiometric compounds and how  they violate law of definite proportions: 

It may be noted that although very useful in the foundation of modern chemistry, the laws of definite proportions is not universally true. There exist non-stoichiometric compounds whose elemental composition can vary from sample to sample. An example is the iron oxide wüstite, which can contain between 0.83 and 0.95 iron atoms for every oxygen atom, and thus contain anywhere between 23% and 25% oxygen.

Now law of definite proportions