So,
Given the following reaction:
We want to find the number of moles of water that can be produced from the reaction of 2.0 moles of hydrogen gas is reacted with 2.5 moles of oxygen gas.
First of all, we need to check which of both reactants is the limiting reactant because that reactant is going to generate the product.
To find the limiting reactant, we're going to use the coefficients of the reaction. The equation tells us that per each two moles of H2, one mol of O2 is needed to react. So,
We have 2 moles of H2, but we need 5 moles of H2 to react according to the reaction.
Since we have less amount of what we really need, then, H2 is the limiting reactant.
Now, the moles of water produced will be:
The answer is 2 moles of H2O.