Which of the following combinations in an aqueous medium will give a red colour or precipitate? — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Group III cations (Fe3+, Al3+, Cr3+) are precipitated as hydroxides using NH4OH in the presence of NH4Cl. Step 2: Adding excess ammonium chloride (NH4Cl) suppresses the ionization of ammonium hydroxide (NH4OH) via the common ion effect of the ammonium ion (NH4+). Step 3: This limits the concentration of hydroxide ions (OH-) to a level that is sufficient to exceed the solubility products (Ksp) of Group III hydroxides but not those of Group IV hydroxides (Ni2+, Co2+, Zn2+, Mn2+), thus preventing their co-precipitation. This matches option (b).