Question

Who‌ ‌gave‌ ‌the‌ ‌statement‌ ‌of‌ ‌law‌ ‌of‌ ‌multiple‌ ‌proportions?‌

Answer 1

Law of multiple proportions is a law under stoichiometry and was given in the year $1803$. This law was given by the same chemist who gave the atomic theory that matter consists of indivisible particles called atoms.

Complete answer:
John Dalton gave the law of multiple proportions. This law is also called Dalton’s Law and was given in the year $1803$. It states that if two elements form more than one compound, then the ratios of masses of the second element which combine with a fixed mass of the first element will always be ratios of small whole numbers.
For example, carbon forms two oxides with the oxygen, i.e., $C{O_2}$ and $CO$. Here a fixed mass of carbon combines with a mass of $32g$ and $16g$ oxygen. These masses of oxygen are in ratio of $2:1$. This ratio is of small whole numbers. Hence, it proves the law of multiple proportions.
The law of multiple proportions is an extension of the law of definite composition, which states that compounds will consist of defined ratios of elements. John Dalton formulated the law of multiple proportions as part of his theory that atoms formed the basic indivisible building block of matter.

Note:
Laws of multiple proportions have certain limitations. It is best demonstrated by simple compounds. if one tried to demonstrate it using compounds like hydrocarbons decane $({C_{10}}{H_{22}})$ and undecane $({C_{11}}{H_{24}})$, the ratio of hydrogen masses for a fixed mass of carbon will be $121:120$. As we can see, this is not a ratio of small whole numbers. This law also fails for polymers and oligomers.