What is the name of the law that states a constant ratio exists between the product and reactant concentrations at equilibrium?

• A. Law of Equilibrium
• B. Le Chatelier's Principle
• C. Avogadro's Law
• D. Hess's Law

Answer: (A)

At fixed temperature, when a reaction reaches equilibrium the concentrations of reactants and products settle into a constant ratio that does not change with time. This ratio, formed by putting the product concentrations in the numerator and the reactant concentrations in the denominator (each raised to its stoichiometric power), is the equilibrium constant. This idea comes from the Law of Mass Action, which is the standard way we describe how far a reaction goes at a given temperature. The magnitude of this constant tells you whether products or reactants are favored at that temperature. The other ideas listed describe different concepts: Le Chatelier’s Principle explains how the system shifts when disturbed, Avogadro’s Law relates gas volume to amount of substance, and Hess’s Law concerns enthalpy changes along different pathways. So the statement about a constant ratio is the hallmark of the equilibrium constant concept, tied to the Law of Mass Action.