Question

what is the concentration of the oxalate ion when the first cation begins to precipitate

Answer 1

What cation(s) do you have? \(Ca^{2+}\) and \(Mg^{2+}\)

There's some missing information there.

Answer 2

sorry haha the other part is:
If a solution contains 0.01 M Ca2+ and 0.01 M NI2+ ions and you want to separate them using aqueous (NH4)2C2O2, which ion will precipitate first as an oxalate, Cac2o4 or Nic2O4?
Ksp CaC2O4=4.75 X 10^-4
Ksp NiC2O4=4.0 X 10^-10

Answer 3

Since Ksp of CaC2O4 > Ksp of NiC2O4, CaC2O4 is more soluble than NiC2O4 and Ni2+ will precipitate first.