If in air there are 21 moles of O2 at 25⁰C and in 1atm, what is the concentration of NO in ppm? (assume that the total concentration of the two Oxides in reaction NO + 1/2 O2 -> NO2 is 5ppm)

Are you sure that this question is complete?

I think yes... but if I was sure maybe I could be able to solve this... Is something missing?

Well, 21 moles of O2 is before of after the reaction?

Is this reaction you are using a reversible reaction?
If so, then you will possibly need the pressure constant of this reaction.

If it's what I'm thinking then the question should say that at equilibrium, there is 21 moles of oxygen gas, because if the reaction is non reversible, then all the oxygen will react with NO to form NO2, then maybe the excess O2 is 21 moles but then, it wouldn't ask for its concentration because it's equal to zero if so.

So, I'll assume that it's a reversible reaction that you meant.

Find the volume that the 21 moles of O2 occupy using the ideal gas equation;



Then, find the concentration of O2 in ppm.

There will be twice that concentration of NO according to your equation, but only if at the beginning the concentration of O2 and NO were in the ratio 1:2 or if the reaction was begun using NO2.