Question

An unknown solution may contain any of these nine ions: CO₃^2–, SO₄^2–, CrO₄^2–, Cl^–, SCN^–, Pb²⁺, Ni²⁺, Cu²⁺, or Fe³⁺.   Using the chemistry you learned in this experiment and a table of solubility rules, list all pairs of ions that could not possibly be present together in the unknown.

Answer 1

Solubility of metal salts in water largely depends on the anions

Carbonates: most of them are insoluble. except Na+,K+ and NH4+ carbonates

Sulfates: Most of them are soluble, except Ba2+and Pb2+

Chromates: Frequently insoluble

Chlorides: Mostly soluble, exceptions: Ag+, Pb2+,Hg2+

Thiocyanate: Mostly soluble, exceptions:Fe3+

Basing on these general trends, following pairs are not possible to existsin aquesous solution as they precipitate out of the solutions

Pb²⁺: CO₃^2–, SO₄^2–, CrO₄^2–, Cl^–, .  

Ni2+: CO₃^2–, CrO₄^2–

Cu2+: CO₃^2–, CrO₄^2–, SCN^–

Fe³⁺: CO₃^2–, CrO₄^2–,