Q.1. Addition of ammonium chloride to a solution containing ferric and magnesium ions is essential for selective precipitation of ferric hydroxide by aqueous ammonia. (1985 - ½ Mark)
Sol. True : Function of ammonium chloride is to suppress the ionisation of NH4OH and thus check the precipitation of Mg(OH)2 because the solubility product of Mg(OH)2 is high.
This is used in salt analysis when 3rd group radicals are precipitated. The group reagent are NH4OH in presence of NH4Cl.
Q.2. From the acidic solution containing copper (+2) and zinc (+2) ions, copper can be selectively precipitated using sodium sulphide. (1987 - 1 Mark)
Sol. True : Ksp of CuS is less than Ksp of ZnS. On passing H2S in acidic medium, the dissociation of H2S is suppressed due to common ion effect and it provides [S2–] which is just sufficient to cross over Ksp of CuS and not Ksp of ZnS. Thus only CuS gets precipitated.