A numerical value, determined by applying the law of mass action to a mixture of reactants and products at any point in time, provides a snapshot of the relative amounts of each species. This value is computed by inserting the starting amounts, rather than equilibrium quantities, into the equilibrium expression.
This computation is a critical step in predicting the direction a reversible process must shift to achieve equilibrium. Comparing this calculated value to the equilibrium constant allows scientists to determine whether a reaction will proceed forward, backward, or is already at equilibrium. Historically, this method has been fundamental in optimizing chemical processes and understanding reaction mechanisms.