How many moles of water are created if 3 moles of hydrogen react completely with excess oxygen?

3.0 moles.

Explanation:

It is a stichiometry problem. The chemical reaction of reacting hydrogen with oxygen to produce water is:

H₂ + 1/2 O₂ → H₂O.

It is clear that 1.0 mole of H₂ reacts with 0.5 mole of O₂ to produce 1.0 mole of water. The ratio of the reacting hydrogen to the produced water is 1:1.

∴ The number of moles of water created from reacting 3.0 moles of hydrogen completely with excess oxygen = 3.0 moles.