Question

The percentage of oxygen in boiled water is more than that in air why

Answer 1

When you boil water , all air dissolved will escape.

So, there is only H2O and some very little impurities as all air has already escaped.

Now in 1 mole H2O, we have 2 moles of H and 1 mole O. So, fraction of oxygen, O = 1/3 = 0.33.
Percentage of O = 33.33%

Note: this is not dissolved oxygen. All dissolved gases escape. This oxygen is the molecular O in water molecule.

Now look at air.
Air has 79% nitrogen and 21 % oxygen,approx . Including O in CO2, CO and other oxides, value can rise upto 25% and not more.

So,
boiled water has more percentage O (33%) than air.

Again Note: The O in boiled water is not available for breathing to fishes or ither organisms. The O is bonded to H to form water.