Consider the reaction: NO(g)+NO2(g)--->N2O3(g) (reversible) for which Kp=2.00 at 20 C. A mixture of NO(g) at?

I am going to help you first, and if you can't figure it out, I will be more detailed. You can solve for the volume... You know Kp, and Kp is solved by putting the partial pressures of the products over the partial products of the reactants. Once you have that solved, PV=nrT should be easy.