Question

Consider a solution that is 2.5×10⁻² M in Fe2+and 1.0×10⁻² M in Mg2+.

What is the remaining concentration of the cation that precipitates first, when the other cation just begins to precipitate?

Answer 1

Lets assume the Mg2+ and Fe2+ are present in the form of carbonates.

1. MgCO₃ Mg²⁺ + CO₃^2-
K_sp = [Mg²⁺][CO₃^2-] =
6.82 x 10^-6 = (1.0 x 10^-2)[CO₃^2-]
[CO₃^2-] = 6.82 x 10^-6 / 1.0 x 10^-2

   ⁼ 6.82 x 10^-4 M requires to precipitate the Mg²⁺ ions

K_sp = [Fe²⁺][CO₃^2-] = 3.07 x 10^-11
3.07 x 10^-11 = [Fe²⁺](6.82 x 10^-4)
[Fe²⁺] = 3.07 x 10^-11 / 6.82 x 10^-4

   ⁼ 4.5 x 10^-8 M when the Mg²⁺ ions precipitates

Now, remaining Fe²⁺

3.07 x 10^-11 = [Fe²⁺ ] [CO₃^2- ]

3.07 x 10^-11 = [Fe²⁺ ] [4.5 x 10^-8 ]

Fe²⁺ = 3.07 x 10^-11 /4.5 x 10^-8 M

= 6.82*10^-4 M