[kjh]

Two formulae are given.
N₂O₅ ⇌ N₂O₃ + O₂, K₁ = 1/4
N₂O₃ ⇌ N₂O + O₂, K₂ = 3
(The equilibrium constants are based on concentrations)
How does the concentration of O₂ change if N₂O₃ is added after the equilibrium is reached?

I thought the concentration of O₂ would decrease because the equilibrium constant of the reverse of the first reaction K_-1 is 4, which is bigger than K₂.
But if you add the reverse of the first reaction and the second reaction, O₂ on both sides cancel out. Does that mean there's no change in concentration?
I am trying to use mass balance equations to find out the answer, but it seems very difficult.

[mjc123]

If you have an equilibrium between N₂O₅ and N₂O₃, and one between N₂O₃ and N₂O, then you have an equilibrium between N₂O₅ and N₂O. Write the equation for this equilibrium and an expression for the equilibrium constant. What is this equal to?
Now suppose you add N₂O₃, and some or all of it disproportionates to N₂O₅ and N₂O, with no net change in the concentration of O₂. What will happen to the concentrations of N₂O₅ and N₂O (and in particular, to the ratio [N₂O]/[N₂O₅])? Is this consistent with the equilibrium? What change, if any, is necessary to re-establish equilibrium?