Based on the equation below how many liters of water can be produced from 8.5 l of oxygen gas. ( assume that hydrogen is in excess)
2h2+ o2= 2h2o

1) Balanced reaction:

2H2 (g) + O2 (g) -> 2H2O(g)

2) mole ratio:

1 mol O2 : 2 mol H2O

3) Ideal gas equation => at constant pressure and temperature, Volumen is proportional to the number of moles.

Then the ration of liters is the same ratio of moles:

1 L O2 : 2 L H2O

4) Use the proportion to find the number of liters of water produced:

8.5 L O2* 2 L H2O / 1L O2 = 17 L

5) 17L of H2O

It changes food into energy that the organism can use.

The mass has escaped into a gas which you can not see so the law stays true.