Question

what is law of constant proportion

Answer 1

the law of definite proportion, sometimes called Proust's law or the law of definite composition, or law of constantcomposition 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.
hope it helps you

Answer 2

Law of constant proportion is the law which states that the given chemical compound always should contain equal elements in both side means reactant side and product side.
example.
C2+2O2=2Co2