A measure of the relative amounts of reactants and products in a reaction system at equilibrium.

• A. Solubility Product Constant
• B. Equilibrium Constant
• C. Le Chatelier's Principle
• D. Phase

Answer: (B)

The measure being described is the equilibrium constant. At a given temperature, for a general reaction aA + bB ⇌ cC + dD, the equilibrium constant K is defined as K = [C]^c [D]^d / [A]^a [B]^b (using activities or concentrations). This single number quantifies how much of the reactants has been converted to products once the system has reached balance. If K is large, products dominate at equilibrium; if K is small, reactants dominate; K around 1 indicates comparable amounts. Temperature affects the value of K, since it reflects the balance of Gibbs free energy for the reaction. The other concepts refer to different ideas: solubility product constant relates to dissolution of a sparingly soluble salt, Le Chatelier's principle describes how a system shifts when disturbed, and phase simply denotes a state of matter.