Ok. But there is no al3+ in the question?
Students may receive a mixture containing Al3+
and may make a mistake according to this scheme (only in the case of Al3+
What about steps 3? We want to precipitate Fe, why bubble in H2S ?
Now iron has appeared? Earlier it was zinc.
Can you answer the question - what should be the concentration of S2-
ions in the solution to precipitate Cu2+
cations from the solution with concentrations, e.g. 0.1 M.? What must be the pH of the solution in order to obtain such a concentration of sulfide ions if we introduce hydrogen sulfide gas into the solution? Assume the maximum concentration of hydrogen sulfide in the acidic medium as 0.05 M.
If you can (and you have to calculate it) then you will understand the idea behind this scheme (for cations of groups II and III.
Otherwise, you have to memorize that.
Note that this scheme was created over 150 years ago and its creators did not understand the rules associated with it then. It was simply an experience of many chemists.