Regardless of how it is prepared or where a sample is collected from, water is always composed of 88.8% oxygen and 11.2% hydrogen by mass. this is an illustration of the

- 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 . 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 =  = 11.2%

mass percentage of O =  = 88.8%

These percentages are always fix, no matter from where the sample of water is collected.