Final answer:
The addition of HCl to a solution of benzoic acid will shift the equilibrium towards the non-dissociated form of benzoic acid, decreasing the concentration of benz- ion.
Step-by-step explanation:
It asks which reagent, when added to a solution of benzoic acid, will shift the equilibrium in the direction of decreasing the concentration of benz- ion. The correct reagent for this is HCl (hydrochloric acid). When HCl is added to the benzoic acid solution, it increases the concentration of H3O+ ions.
This increase in hydronium ions leads to the reaction with the benz- ion (benzoate anion), forming more of the non-dissociated benzoic acid, thus reducing the concentration of the benz- ion in the solution.