Le Chatelier's Principle in Equilibrium

Steps:

• The reaction is at equilibrium: 2NO₂(g) ⇌ 2NO(g) + O₂(g); NO₂ is brown, while NO and O₂ are colorless.
• The pale brown color indicates partial decomposition with some NO₂ present.
• Adding O₂ increases the concentration of a product.
• Equilibrium shifts left to consume excess O₂, producing more NO₂.

Why A is correct:

• Shifting left increases NO₂ concentration, darkening the brown color per Le Chatelier's principle.

Why the others are wrong:

• B: Equilibrium shifts toward more NO₂, not less, so it does not pale.
• C: NO₂ remains and increases, preventing colorlessness.
• D: Adding O₂ perturbs equilibrium, causing a shift and color change.

Final answer: A