Question

The equilibrium concentration of hydroxide ion in a saturated iron(iii) hydroxide solution is

Answer 1

when:

Fe(OH)3 ↔  Fe3+    +    3OH-

so, each 1 mol Fe3+ →  3 mol OH-

and when [OH] = 1.2 x 10^-5

∴ [Fe3+] = (1.2 x 10^-5) / 3

               =  4 x 10^-6

when Keq = [Fe3+][OH]^3

                  = (4 x 10^-6) (1.2 x 10^-5)^3

    ∴Keq     = 6.9 x 10^-21