An aqueous mixture contains 0.010 M Fe²⁺ and 0.010 M Mn²⁺ at 25 °C. The solubility-product constants are Ksp[Fe(OH)2]=4.9×10−17 and Ksp[Mn(OH)2]=1.6×10−13. The pH of the solution is slowly increased by the addition of NaOH(aq).
Which of the following pH values most likely corresponds to a condition in which Fe(OH)₂(s) has started to precipitate but Mn(OH)₂(s) is still completely dissolved?
a) 6.0
b) 12.0
c) 7.5
d) 9.0