Law of definite proportion

The law of definite proportions, also known law of definite composition, states that regardless of the amount, a pure compound always contains the same elements in the same proportions by mass. Law of multiple proportions, also known as Dalton s Law, states that when one element combines with another to form more than one compound, the mass rations of the elements in the compounds are simple whole numbers of each other.

LAW OF DEFINITION PROPORTIONS EXAMPLE
The law of definite proportions says water will always contain 1/9 hydrogen and 8/9 oxygen by mass.

EXCEPTIONS TO THE LAW OF DEFINITE PROPORTIONS
Although the law of definite proportions is useful in chemistry, there are exceptions to the rule. Some compounds are non-stoichiometric in nature, meaning their elemental composition varies from one sample to another. For example, wustite is a type of iron oxide with an elemental composition varying between 0.83 and 0.95 iron atoms for each oxygen atom (23%-25% oxygen by mass). It ideal formula is FeO, but the crystal structure is such that there are variations. The formula is written Fe0.95O.

Also, isotopic composition of an element sample varies according to its source. This means the mass of a pure stoichiometric compound will be slightly different depending on its origin.

Polymers also vary in element composition by mass, although they are not considered true chemical compounds in the strictest chemical sense.