The complex ion [Fe H2O 6]3+ can undergo hydrolysis in water, which means it can lose protons H+ and form hydroxide ions OH- in the process. The hydrolysis reaction can be represented as follows:[Fe H2O 6]3+ + H2O [Fe H2O 5 OH ]2+ + H3O+The equilibrium constant for this reaction is given by:K = [ [Fe H2O 5 OH ]2+ ] [H3O+] / [ [Fe H2O 6]3+ ]When the pH is changed from 2.0 to 3.5, the concentration of H3O+ ions in the solution will decrease. We can calculate the concentration of H3O+ ions at each pH using the formula:[H3O+] = 10^-pH At pH 2.0:[H3O+] = 10^-2 = 0.01 MAt pH 3.5:[H3O+] = 10^-3.5 3.16 x 10^-4 MAs the concentration of H3O+ ions decreases, the equilibrium will shift to the right to compensate for the change, according to Le Chatelier's principle. This means that more [Fe H2O 5 OH ]2+ complex ions will be formed, and the concentration of [Fe H2O 6]3+ complex ions will decrease.In conclusion, when the pH is changed from 2.0 to 3.5, the formation of the complex ion [Fe H2O 6]3+ will be less favored, and the concentration of [Fe H2O 6]3+ will decrease.