Question

law of conversation of proportion

Answer 1

Proust's law or the law of definite composition, or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation.

Answer 2

A French chemist Joseph Proust states this law in the following way.
"A chemical compounds always contains the same element combined together in the same proportion by mass."