Question

A solution contains 0.0450 M Co2 and 0.0350 M Ni2 . What is the concentration of the first cation when the second starts to precipitate as Na2CO3 is added to the solution?

Answer 1

The first thing we have to do is to get the values of ksp of CoCO₃ and NiCO₃ which are:

CoCO₃ ;1.4 x10^-13

NiCO₃ ;1.42 x 10^-7

Now we can know which cathion will precipitate first, it will be the one with the smallest ksp value, in this case the Co will precipitate first and the Ni will precipitate later.

Now we have to make calculations in order to know the ammount of CO₃ present.

1.42 x 10^-7 = Ni * CO₃

1.42 x 10^-7 = 0.035 * CO₃

CO₃ = 4.06 x 10^-6 M

Now we can know the ammount of Co when the second starts to precipitate, we just have to use the CO₃ value to find the ammount of Co corresponding to its solubility.

1.4 x 10^-13 = Co * 4.06 x 10^-6 M

Co = 3.45 x 10^-8 M