The equilibrium constant for a reaction at a fixed temperature is defined as:

• A. The ratio of product concentrations to reactant concentrations at any time
• B. The sum of product and reactant concentrations at equilibrium
• C. The ratio of product concentrations to reactant concentrations at equilibrium
• D. The rate constant of the reaction

Answer: (C)

The equilibrium constant is defined at a given temperature as the ratio of product concentrations to reactant concentrations when the system has reached equilibrium. For a general reaction aA + bB ⇌ cC + dD, it’s K_eq = ([C]^c [D]^d)/([A]^a [B]^b). This uses the concentrations (or activities) of the species at equilibrium and stays the same as long as the temperature is fixed. It isn’t the sum of concentrations, nor the rate constant of the reaction—the rate constant describes the speed of the process, not the final balance of species. If you know K_eq, you can infer which side is favored at that temperature (large K_eq favors products, small K_eq favors reactants).