Question

equilibrium constant for the hydrolysis of fe3+ to fe(OH)2+ and H+ os 6.5x10^-3. What is the maximum pH at which at least 95% of the total Fe3+ in a dilute solution exists as Fe3+?

Answer 1

Given:

• The equilibrium constant (k) for the reaction given below is 6.5*10⁻³                                                   Fe³⁺ + H₂O -----> Fe(OH)²⁺ + H⁺
• The degree of hydrolysis of Fe³⁺ (h) is 0.05 (1-95/100)

To find:

The pH at which 95% of Fe³⁺ is not hydrolyzed.

Solution:

• Let initial concentration of Fe³⁺ be C.
• At equilibrium, [Fe³⁺] = 0.95C, [Fe(OH)²⁺] = [H⁺] = 0.05C
• k = [Fe(OH)²⁺].[H⁺]/[Fe³⁺] = 0.05*0.05C/0.95 = 6.5*10⁻³ ⇒ C = 2.47 M
• [H⁺] = 0.05C = 0.1235 M
• pH of the solution = -log([H⁺]) = 0.91

Answer:

The pH at which 95% of Fe³⁺ is not hydrolyzed is equal to 0.91