Question

The law of multiple proportions talk about mass whereas the law of definite proportion talks about weight, why?

Answer 1

The law of multiple proportions states: when two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.
The law of definite proportions, sometimes called Proust's law or the law of definite composition, states that a chemical compound always contains exactly the same proportion of elements by mass.