Question

A solution has a pH of 7.30 and an alkalinity of 25 mg/L as CaCO3. Essentially all of the alkalinity is contributed by the carbonate system. The water also contains 10–4.00 M Fe2+ (5.5 mg Fe2+/L). To remove the iron, oxygen is injected into the solution, converting the Fe2+ to Fe3+; the Fe3+ then precipitates as ferric hydroxide (Fe(OH)3(s)) according to the following reaction:

Fe2+ + 0.25 O2(aq) + 2.5 H2O ⇌ Fe(OH)3(s) + 2 H+

What is the pH of the water after the reactions are complete?

Answer 1

pH of solution = 7.3

so that, [H^+] = 10^-7.3 M

after the reaction

Fe2+ + 0.25 O2(aq) + 2.5 H2O ⇌ Fe(OH)3(s) + 2 H+

concentration of Fe^2+ = 10^-4 M (or) mole/L

1 mole Fe^2+ = 2 mole H^+

from the above

concentration of H^+ generated = 2*10^-4 M

Total H^+ concentration in solution = (2*10^-4 )+(10^-7.3)

                                      = 2*10^-4 M

pH = -log(H+)

     = -log(2*10^-4)

     = 3.7