Question

state and explain law of constant proportion with an example

Answer 1

The law of definite proportion, sometimes called 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. For example oxygen makes up about 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass.

Answer 2

a compound is always having a fixed proportion of elements by mass or
the elements which are present in a compound is always having a fixed mass ratio irrespective of the mass of the compound