Answer
See work below.
Work Step by Step
To choose a qualitative procedure, we will need to consider which group the cation belongs. $Zn^{2+}$ belongs to group 3, since $ZnS$ is a base-insoluble sulfide. On the other hand, $Cd^{2+}$ can be classified as a Group 2 cation, as $CdS$ is an acid-insoluble sulfide. Therefore, we can utilize the following procedure:
From the initial solution, we make an addition of a certain volume of $HCl$, so that the pH of the solution is approximately 1. Then, bubble $H_2S$ through the solution; the precipitate formed is $CdS$. Finally, we filter the heterogeneous mixture: the residue on the filter paper is $CdS$, while the filtrate contains $Zn^{2+}$ as a predominant species, with little amount of $Cd^{2+}$ and $S^{2-}$.
