A student was given a solid containing a mixture of nitrate salts. The sample completely dissolved in water, and upon addition of dilute HCl, no precipitate formed. The pH was lowered to about 1 and H2S was bubbled through the solution. No precipitate formed. The pH was adjusted to 8 and H2S was again bubbled in. This time, a precipitate formed. Which compounds might have been present in the unknown?
What nitrates did you have? I assume you have list from which to choose. You could have
Ni, Co, Mn, Fe, Al, Cr, Zn
Na, Ca, Zn, Al, Mn
adw
To determine the compounds that might have been present in the unknown mixture, we need to analyze the reactions that occurred during the experiment.
1. Complete dissolution in water and no precipitate with dilute HCl:
This suggests that the unknown mixture does not contain any chloride salts, as they would have formed a precipitate upon the addition of dilute HCl.
2. No precipitate formation with H2S at a low pH (around 1):
At low pH, only certain metal sulfides can precipitate when H2S is bubbled through the solution. Since no precipitate formed, it suggests that metals with sulfide insolubility constants larger than the concentration of sulfide ions at low pH are not present.
3. Precipitate formation with H2S at pH 8:
At pH 8, more metal sulfides can precipitate when H2S is bubbled through the solution. The formation of a precipitate suggests that metals with sulfide insolubility constants smaller than the concentration of sulfide ions at pH 8 are present in the unknown mixture.
Possible compounds that might have been present can be determined by considering the solubility rules and the sulfide-insolubility constant values for various metal sulfides. Generally, low-valence metal sulfides tend to be insoluble, while high-valence metal sulfides tend to be more soluble.
Here are some possible compounds that could have been present based on the observations:
- Lead sulfide (PbS)
- Copper sulfide (CuS)
- Silver sulfide (Ag2S)
Please note that this is not an exhaustive list, and other compounds may also be present depending on the experimental conditions and the specific properties of the unknown mixture. Further testing and analysis (such as spectroscopy or additional chemical tests) might be necessary to confirm the presence of specific compounds.