Group III cation analysis question?

Question

After the separation of the precipitates NiS and CoS, the solution containing the other grp III cations were treated with 4M HNO3 & the solution was evaporated to a moist residue. Then the residue were dissolved with 1ml of water & the solution was transfered to a test tube. 4M NaOH were added until precipitate appeared, then 10 drops of 4M NaOH were added in excess. 6 drops of 3% H2O2 were added, then the solution was heated in a hot water bath for 5 min. The mixture was then separated: the residue washed with 10 drops water to which has been added 1drop of 4M NaOH & the centrifugate reserved for succeeding cation tests. The residue were then treated with 1mL of 4M HNO3 & 2drops 1 NaNO2. The mixture was stirred & heated in a water bath. The residue was separated from the solution & discarded.
questions:
(1) what is the reason for the addition of "4M HNO3 & evaporation of the solution to a moist residue"?
(2) why add excess NaOH?
(3) What is the residue discarded in the last part?

Answer 1

1) To dissolve the sulphides as nitrates

2) To chelate the ions

3) Sulphur.

Answer 2

Since you said chromium ion, meaning the chromium atom is dissolved into OH and H ions. Ammonium Chloride is readily dissociated into NH3 and HCl. HCl can dissociate into its respective ions H+ and Cl-. The Cl- will bond with the Cr3+ to give u CrCl3, though this will not be the main product formed.