3) If 2.5 moles of H2O are produced, how many moles of hydrogen gas must be used?

2.5 moles of H2

Explanation:

First, let us write a balanced equation for the reaction of Hydrogen to produce water. This is shown below:

2H2 + O2 —> 2H2O

From the equation above,

2 moles of H2 produced 2 moles of H2O.

Therefore, 2.5 moles of H2 will produce 2.5 moles of H2O