Which factor does not affect the equilibrium constant?

The equilibrium constant, denoted as K, is a value that reflects the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. It is important to recognize that the equilibrium constant is influenced by certain factors while remaining unchanged by others.

In this context, a catalyst is a substance that increases the rate of a reaction without being consumed in the process. It achieves this by providing an alternative pathway with a lower activation energy for the reaction to proceed. However, a catalyst does not alter the equilibrium position of a reaction or change the concentrations of reactants and products at equilibrium — it only helps the system reach equilibrium faster.

Therefore, the equilibrium constant remains unaffected by the presence of a catalyst, making it the correct choice in this question. In contrast, temperature, pressure, and concentration of reactants all have significant impacts on the equilibrium constant. For example, changes in temperature can shift the equilibrium and change the value of K, while changes in concentration or pressure can alter the equilibrium position, but will not change K directly.