Answer:
So the answer would be 10 moles
Explanation:
1) Start with the molecular formula for water:
2) If there are 10 moles of water use a mole ratio to calculate the moles of oxygen it would produce.
(This question is... interesting... since they chose an element that is diatomic in free state so It could TECHNICALLY be two answers, moles of O or moles of )
The mole ratio is 1 moles of to 1 moles of O. This is because the coefficient for oxygen in water is simple 1, so the ratio is 1:1.
3) that means if 10 moles of water decompose, they decompose into 10 moles of and 10 moles of O.
Extra:
About what I was saying before about the question being slightly interesting:
10 moles of pure oxygen is produced but free state oxygen exists as so it could possibly be 10 OR 5! However, notice it says elements. This leads me to believe the answer is 10 (monatomic oxygen) instead of 5 (free state/diatomic oxygen).
I hope this helps!