Final answer:
According to Le Chatelier's principle, adding more Fe(NO3)3, which dissociates into Fe³⁺ ions, increases the concentration of Fe³⁺ ions.
Step-by-step explanation:
Applying Le Chatelier's principle to the endothermic reaction Fe³⁺(aq) + Cl⁻(aq) → FeCl²⁺(aq), we can predict how the equilibrium concentration of the complex ion FeCl²⁺ will change when Fe(NO₃)₃ is added. Since Fe(NO₃)₃ dissociates in water to form additional Fe³⁺ ions, adding it to the mixture will increase the concentration of the Fe³⁺ ions.
According to Le Chatelier's principle, the system will counteract this by shifting the equilibrium to the right, which increases the production of FeCl²⁺ to decrease the concentration of Fe³⁺, thus increasing the equilibrium concentration of the complex ion FeCl²⁺.
For this particular reaction, we will be able to see that this has happened because the solution will become a darker green color.