If the equilibrium constant K for a reaction is much greater than 1, which statement is true about the equilibrium mixture?

• A. Predominantly products
• B. Predominantly reactants
• C. Exactly half and half
• D. Cannot be determined

Answer: (A)

The main idea is that the equilibrium constant tells us how the amounts of products and reactants compare at equilibrium. For a reaction, K is the ratio of product concentrations (each raised to its coefficient) to reactant concentrations. When K is much greater than 1, this ratio is large, meaning the concentrations of products are much higher than those of reactants at equilibrium. So the system sits mostly on the product side, with most of the reactants converted to products, though a small amount of reactants can remain depending on the exact conditions.

In other words, the equilibrium mixture is predominantly products. If the ratio had been much less than 1, we'd expect mostly reactants; if it were about 1, we'd have roughly comparable amounts; and if you couldn’t determine anything from K, that would imply no clear tendency, which isn’t the case here because a large K clearly favors products.