An unknown solid is entirely soluble in water. On addition of dilute HCl, a precipitate forms. After the precipitate is filtered off, the pH is adjusted to about 1 and H2S is bubbled in; a precipitate again forms. After filtering off this precipitate, the pH is adjusted to 8 and H2S is again added; no precipitate forms. No precipitate forms upon addition of (NH4)2HPO4. The remaining solution shows a yellow color in a flame test (see Figure 7.21). Based on these observations, which of the following compounds might be present, which are definitely present, and which are definitely absent: CdS, Pb(NO3)2, HgO, ZnSO4, Cd(NO3)2, and Na2SO4?

Figure 7.21