Final answer:
Adding HCl to the equilibrium of HF dissociation increases the H₃O+ concentration, causing the equilibrium to shift to the left, which is justified by Le Chatelier's principle to reduce the stress by producing more HF.
Step-by-step explanation:
When HCl is added to the equilibrium system HF(aq) + H₂O(l) ↔ F⁻(aq) + H₃O⁺ (aq), with a pKa of 3.18, there is an increase in the concentration of H₃O⁺ ions. According to Le Chatelier's principle, this addition will cause a stress on the system, and the equilibrium will shift to counteract this stress. Since HCl is a strong acid, it dissociates completely in solution, adding more H₃O⁺ ions and thus increasing the hydronium ion concentration. The equilibrium will respond by shifting to the left, trying to decrease the concentration of H₃O⁺ ions by consuming them and producing more HF in the process. This shift restores a new equilibrium state, adhering to the constant ratio defined by the equilibrium constant Ka for the weak acid HF.