first, we need to get POH value & we can get it from this formula:
PH + POH = 14
when we have PH = 8.59
∴ POH = 14 - 8.59 = 5.41
then, we need to get [OH-] & we can get it using POH:
POH = -㏒[OH]
5.41 = -㏒[OH]
∴ [OH] = 3.89 x 10^-6
when Ksp = [Fe2-][OH-]^2
and when we have Ksp = 4.87 x 10^-17
so, by substitution:
4.87 x 10^-17 = [Fe2-] * (3.89 x 10 ^-6)^2
∴[Fe2-] = 3.22 x 10^-6 M
∴ above 3.22 x 10^-6 M of Fe2+ , Fe(OH)2 will precipitate
