Analytical Chemistry Question

Question

A solution contains 0.0450 M Co2+ and 0.0200 M Ni2+. What is the concentration of the first cation when the second cation starts to precipitate as Na2CO3 is added to the solution?

Answer 1

For this question, we need to know the Ksp values for CoCO₃ and for NiCO₃. Looking them up provided the following values:

Ksp CoCO₃ = 1x10^-10

Ksp NiCO₃ = 6.6x10^-9

Next, we determine the [CO₃] from Na₂CO₃ needed to precipitate each cation:

CoCO₃(s) => Co²⁺ + CO₃^2-

Ksp = [Co²⁺][ CO₃^2-]

1x10^-10 = (0.0450)(x)

x = 2.2x10^-9 M Na₂CO₃

NiCO₃(s) => Ni²⁺ + CO₃^2-

Ksp = [Ni²⁺][ CO₃^2-]

6.6x10^-9 = (0.0200)(x)

x = 3.3x10^-7

So, the Co²⁺ will precipitate first before the Ni²⁺. At this point, I'm unclear as to what the question is asking since not sure what is meant by the first cation and which is the second cation? Is Co²⁺ considered the first cation? Usually, they ask what is the concentration of the "other" cation when the first cation starts to precipitate. If that's the case, then ...

6.6x10^-9 = [Ni²⁺][CO₃^2-]

6.6x10^-9 = [Ni²⁺][3.3x10^-7]

[Ni²⁺] = 0.020 M which is exactly what was there to begin with, which makes sense in light of the Ksp values for the 2 precipitates.

** Maybe I'm missing something simple in the question, but it really doesn't make a lot of sense to me the way it is worded. Still unclear which is the "first" and which is the "second" cation.