In order to solve this question, we can use the concept of molar ratio, which means the amount of reactant is needed in order to produce a certain amount of product. In our question we have:
2 C2H2 + 5 O2 -> 4 CO2 + 2 H2O
The molar ratio between O2 and H2O is 5:2 (we use only the numbers in front of the compound), so that means that for every 5 moles of O2 used, we will have 2 moles of H2O being produce, now we can use this information to calculate how many moles of H2O will be produced from 3.80 moles of O2
5 O2 = 2 H2O
3.80 O2 = x H2O
x = 1.52 moles of H2O are produced from 3.80 moles of O2
