[...] Note ppm is taken on a molar basis for gases. [...]
And I don't see clearly how to compute an "equilibrium" between 5ppm NO+NO2
in air at 25°C 1atm, while measured concentration in urban area is like 0.01ppm, suggesting that the equilibrium concentration is far less.
In other words: the given reaction determines nothing. Both NO and NO2
concentrations result from the production and destruction speeds far from equilibrium, and if some sub-ppb equilibrium was reached, then the governing reaction would be with N2
But, well, never risk an answer more clever than the question.
So to provide the expected answer, get the equilibrium constant (or entropy of formation, or Gibbs, or µ) versus the elements, deduce it for the NO versus NO2
reaction, and paste the false value where desired to be a good student.