Problem: A sample of pure NO2 gas heated to 1000 K decomposes: 2NO2(g) ⇌ 2NOg) + O2(g)       Kp = 158Analysis shows that the partial pressure of O2 is 0.25 atm at equilibrium. Calculate the pressure of NO2(g) at equilibrium.A. 0.090 atmB. 0.015 atmC. 0.020 atmD. 0.038 atmE. 0.075 atm

We’re being asked to determine the pressure of NO₂(g) at equilibrium for the following reaction:

2 NO₂(g) ⇌ 2 NO(g) + O₂(g)       K_p = 158

Recall that the equilibrium constant is the ratio of the products and reactants. 

We use K_p when dealing with pressure and K_c when dealing with concentration:

$\boxed{{\mathbf{K}}_{\mathbf{p}}\mathbf{=}\frac{{\mathbf{P}}_{\mathbf{products}}}{{\mathbf{P}}_{\mathbf{reactants}}}}$     $\boxed{{\mathbf{K}}_{\mathbf{c}}\mathbf{=}\frac{\mathbf{\left[}\mathbf{products}\mathbf{\right]}}{\mathbf{\left[}\mathbf{reactants}\mathbf{\right]}}}$

Note that solid and liquid compounds are ignored in the equilibrium expression.