Consider the reaction represented by the (unbalanced) equation: NH3 + O2→NO + H2O

For every 1.00 mol of NH3 that reacts, how many mols of O2 is required?

4NH3 + 5 O2 --> 4 NO + 6 H2O

Since we have 4mol NH3 / 5mol O2, so with every 1.00mol NH3 will require 1.25 mol O2

It's like this:

(1.00 mol NH3 x 5 mol O2) / 4 mol NH3 = 1.25 mol O2