In the given chemical equation, 4 moles of nh3 gas react with 5 moles of oxygen to form 6moles of water vapours (h2o). Hence each mole of nh3 contributes to production of 1.5 moles of water vapours.
therefore, the rate of formation of h2o is 1.5*0.5 ms-1, i.e 0.75 moles/sec