what's the concentration ??? chemistry question?

Question

Answer 1

3.0 M of NH3 would react completely with 3.0 x (5/4) = 3.75 M of O2, but there is more O2 present than that, so O2 is in excess and NH3 is the limiting reactant.

(3.0 M NH3) x (6 mol H2O / 4 mol NH3) = 4.5 M H2O(g)