Answer:- Law of definite proportions.
Explanations:- The chemical composition of a compound is always constant means it contains the same elements in exactly same proportions by mass. This is called "Law of definite proportions."
Molecular formula of water is [tex]H_2O[/tex] . It has two hydrogen atoms and one oxygen atom. Atomic mass of H is 1.008 and that of oxygen atom is 15.999.
Molar mass of water = 2(1.008) + 15.999 = 18.015
mass percentage of H = [tex](\frac{2.016}{18.015})100[/tex] = 11.2%
mass percentage of O = [tex](\frac{15.999}{18.015})100[/tex] = 88.8%
These percentages are always fix, no matter from where the sample of water is collected.