Answer
See answer below.
Work Step by Step
a) Number of moles of ammonia:
$750.\ g\div 17.03\ g/mol=44.04\ mol$
Number of moles of oxygen:
$750.\ g\div 32.00\ g/mol=23.44\ mol$, it's the limiting reactant.
Mass of water:
$23.44\ mol\times 6/5 \times 18.015\ g/mol=506.70\ g$
b) Mass of oxygen:
$44.04\ mol\times 5/4 \times 32.00\ g/mol=1761.6\ g$