How many moles of oxygen (o2) are required to completely react with 27.4 mol of h2?

This reaction can be written as
H2 + O2 => H2O. 
Then, balanced, as
2H2 + O2 = 2H2O

There we see that we need 2 times more mol of H2 than O2. So mol of O2 is 1/2 mol of H2. That is 27.4 mol / 2 = 13.7 mol.

It has to be b i hope this