Final answer:
The correct equilibrium constant expression for the reaction C(s) + O₂ (g) → CO₂ (g) is [CO₂], because pure solids do not appear in the equilibrium expression and the concentration of solid carbon is incorporated into the constant value.
Step-by-step explanation:
The equilibrium constant expression for the reversible reaction C(s) + O₂ (g) → CO₂ (g) is represented by considering the activities of the reactants and the products in their gaseous state since pure solids and liquids do not appear in the equilibrium expression.
Hence, the correct equilibrium constant expression for the given system is [CO₂]. This is because the activity of the solid carbon (C) does not change during the reaction and is therefore incorporated into the constant value, whereas the activity of O₂ is represented through its concentration or partial pressure in the gaseous state.
Therefore, the only varying component that affects equilibrium is the concentration of CO₂ in the gas phase, and that is why it is the only term that appears in the equilibrium constant expression in this case.