Neco 2017 Chemistry Question 50

A laboratory analysis is performed on an unknown aqueous mixture that may contain any of the following cations: \(\text{Ag}^+\), \(\text{Pb}^{2+}\), \(\text{Hg}_2^{2+}\), \(\text{Cu}^{2+}\). The following sequential tests are carried out:\n1. Dilute HCl is added to the sample. A white precipitate forms which is insoluble in excess HCl.\n2. The precipitate is filtered off. The filtrate is then acidified with concentrated HNO₃ and treated with a potassium chromate solution, producing a bright red precipitate.\n3. The remaining filtrate (after step 2) is made alkaline with NaOH; a blue precipitate appears which dissolves in excess NH₃ to give a deep‑blue solution.\nIdentify all cations present in the original mixture.