Question

which pair will show common ion effect 1) BaCl2 + Ba(NO3)2 2) NaCl + HCl 3) NH4OH + NH4Cl 4) NaCN + KCN

Answer 1

Common ion effect is normally applied in selective precipitation of ions which involves a technique of separating ions in an aqueous solution by using a reagent that precipitates one or more of the ions. addition of common ions in a solution that is already having that given ion normally leads to a formation of a precipitate. The common ion normally decreases the solubility of a slightly insoluble salt.

So for these pairs you have given, I would like to advice you that all of them show common ion effect:

1) BaCl2 + Ba(NO3)2 - common ion is Ba2+

2) NaCl + HCl3 - common ion is Cl-

3) NH4OH + NH4Cl - common ion is NH4+

4) NaCN + KCN - common ion is CN-