Question

Separate samples of a solution of an unknown substance salt were treated with solutions of HCL, H₂SO₄, and NaOH. Precipitate forms in all three cases. Which of the following cations could the solution contain? Li⁺ Hg²⁺ or Ba²⁺? Generate a matrix that will explain your conclusion (the matrix should explain all).

Answer 1

First, get solubility rules:

- Most alkali metal (group I metals) are always soluble

- Cations/Anions such as NH4+, HCO3-; ClO3-, NO3- are always soluble

- Most halides are soluble, F-,Cl-,Br-,I-, exceptions = Ag+, Hg2+2, Pb+2 compounds

- Most sulfates are soluble (SO4-2); exceptions = Ag+, Ca+2, Sr+2, Ba+2, Hg2+2, Pb+2 compounds

- In general (except all alkali metals) these ions will NOT be soluble: PO4-3, CrO4-2, S-2, CO3-2

a)

Li+ is an alkali metal, will not form any precipitate with Cl-, SO4-2 or OH-

therefore, Li+ can't be present

Hg2+ is likely to form:

Hg2Cl2; Hg2SO4; Hg2(OH)2

therefore, Hg2+2 is definetivly present

c)

Ba2+ ion will:

form precipitate with SO4-2 and OH- but will not form with Cl- ion

BaCl2 is soluble

therefore, the onyl species precipitating is Mercury , Hg2+2