For the gas-phase equilibrium 2 NO2(g) ⇌ N2O4(g), at equilibrium [NO2] = 0.200 M and [N2O4] = 0.100 M. Calculate Kc.

For the gas-phase equilibrium 2 NO2(g) ⇌ N2O4(g), at equilibrium [NO2] = 0.200 M and [N2O4] = 0.100 M. Calculate Kc.

• A. 0.5
• B. 1
• C. 2.5
• D. 10

Answer: (C)

The key idea is how to write the equilibrium constant expression for a gas-phase reaction. For 2 NO2 ⇌ N2O4, the expression is Kc = [N2O4] / [NO2]^2, because two NO2 molecules combine to form one N2O4. Plug in the given concentrations: [N2O4] = 0.100 M and [NO2] = 0.200 M. So Kc = 0.100 / (0.200)^2 = 0.100 / 0.0400 = 2.5. Thus the value of the equilibrium constant is 2.5, indicating the equilibrium lies somewhat toward the product side.

The key idea is how to write the equilibrium constant expression for a gas-phase reaction. For 2 NO2 ⇌ N2O4, the expression is Kc = [N2O4] / [NO2]^2, because two NO2 molecules combine to form one N2O4.

Plug in the given concentrations: [N2O4] = 0.100 M and [NO2] = 0.200 M. So Kc = 0.100 / (0.200)^2 = 0.100 / 0.0400 = 2.5.

Thus the value of the equilibrium constant is 2.5, indicating the equilibrium lies somewhat toward the product side.