[Raww]

A solution contains 7.16E-3 M potassium carbonate and 1.08E-2 M sodium sulfide.
Solid iron(II) acetate is added slowly to this mixture.

What is the concentration of sulfide ion when carbonate ion begins to precipitate?
[sulfide] = ____ M

This seems to be some sort of titration problem, either way I am having a tough time figuring it out.

[Borek]

No, this is about solubility products.