[vared8]

Consider the following reaction, which takes place in an autoclave at 2500C and 800 atm. NH3(g) +7/2O2--> NO2(g) +  3/2H2O2(g)
Into the reaction vessel has been placed 200L of NH3(g) and 120L of O2(g). The reaction is allowed to go to completion. Determine the quantity, in moles, of the gas that remains un-reacted.

[Borek]

Please read forum rules.

Pretty simple stoichiometry. Find limiting reagent. Hint: use Avogadro's law.

[AWK]

Avogadro law is useless at this pressure and temperature. Moreover, at this temperature  more thermodynamically stable is NO, not NO2, and ammonia practically does not exist (it decomposes to N2 + H2), H2O2 will decompose to H2O and O2.

Moreover, you need temperature and pressure of gases used in this reaction because it is practically impossible to pump gases at this temperature.
Then using your assumption, nonsense reaction, and absolutely unusable in this case the ideal gas law, this is in fact a simple stoichiometry.