J.R. S. answered 01/16/19
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4NH3 + 7O2. ==> 4NO2 + 6H2O balanced equation
moles O2 used = 8.5 g O2 x 1 mole/32 g = 0.266 moles O2
moles NH3 needed = 0.266 mol O2 x 4 mol NH3/7 mol O2 = 0.152 moles
At STP, 0.152 moles x 22.4 L/mole = 3.4 liters of NH3