Question

Consider a solution that is 2.1×10^?2 M in Fe2+and 1.8×10^?2 M in Mg2+.

I know that the Fe2+ was the first to precipitate and that the minimum concentration of K2CO3 is

1.5×10?9

M

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

Answer 1

first consider Fe+2 ions

FeC03 ---> Fe+2 + C032-

Ksp = [Fe+2] [C032-]

given

[Fe+2] = 2.1 x 10-2

also

ksp of FeC03 is 3.07 x 10-11

so

3.07 x 10-11 = 2.1 x 10-2 x [C032-]

[C032-] = 1.462 x 10-9 M

so

Fe+2 starts to precipitate when [C032-] is 1.462 x 10-9 M

now

consider Mg+2

MgC03 ---> Mg+2 + C032-

Ksp = [Mg+2] [C032-]

given

[Mg+2] = 1.8 x 10-2

also

ksp for MgC03 is 6.82 x 10-6

so

6.82 x 10-6 = 1.8 x 10-2 x [C032-]

[C032-] = 3.79 x `10-4 M

so

Mg+2 starts to precipitate when [C032-] is 3.79 x 10-4 M

looking at both of them

we get

Fe+2 precipitates first at 1.462 x 10-9 M

so

the minimum conc of potasium carbonate for precipitation is 1.462 x 10-9 M

b)

from the above we get

Mg+2 starts to precipitate when [C032-] is 3.79 x 10-4 M

so

substitute this value in the ksp of FeCO3

we get

kSp = [Fe+2] [C032-]

3.07 x 10-11 = [Fe+2] x 3.79 x 10-4

[Fe+2] = 8.1 x 10-8

so

the conc of Fe+2 when Mg+2 starts to precipitate is 8.1 x 10-8 M