If 0.050 mol/L of NO2 (g) exists at equilibrium, what is the concentration of N2O5 (g) at equilibrium (with no products present)?

_                              3NO₂(g) ⇌ N₂O₅(g) + NO(g)   Kc = 1.0 × 10⁻¹¹
_Initial (mol/L)               x              0             0
_Change (mol/L)        -3y            +y           +y
_Equilibrium (mol/L)  0.050           y            y

At equilibrium:
Kc = [N₂O₅] [NO] / [NO₂]³
1.0 × 10⁻¹¹ = y² / (0.050)³
y = √[(1.0 × 10⁻¹¹) × (0.050)³]
y = 3.5 × 10⁻⁸

[N₂O₅] at equilibrium = 3.5 × 10⁻⁸ mol/L