If after precipitation of the Group I cations as chlorides by the reaction with HCl, nearly all of the precipitate dissolves in the hot water, what ion is most probably present and which ions are most probably absent?
Pb ion almost certainly is present while Ag and Hg(I) ions probably are absent.
If nearly all of the precipitate dissolves in hot water after precipitation of the Group I cations as chlorides by the reaction with HCl, it indicates the presence of the silver ion (Ag+). This is because silver chloride (AgCl) is soluble in hot water but insoluble in cold water. Therefore, if the precipitate dissolves in hot water, it suggests the formation of AgCl initially.
On the other hand, the ions that are most probably absent in the solution are the chloride ions (Cl-), as they have reacted with the Group I cations to form the insoluble silver chloride precipitate. Hence, after the precipitation and subsequent dissolution in hot water, the chloride ions are depleted from the solution.
To determine which ion is most likely present and which ions are most likely absent in the given scenario, we need a basic understanding of the properties of Group I cations and their behavior with HCl and hot water.
Group I cations consist of silver (Ag+), lead (Pb2+), and mercury (Hg2+). When these cations react with HCl, they form insoluble chlorides, namely, silver chloride (AgCl), lead chloride (PbCl2), and mercury(I) chloride (Hg2Cl2).
Now, if nearly all of the precipitate (formed by the reaction with HCl) dissolves in hot water, we can make the following conclusions:
1. The ion that is most probably present:
Since the precipitate dissolves, it indicates the presence of chloride ions (Cl-) in the solution. Therefore, the ion that is most likely present is the chloride ion (Cl-).
2. The ions that are most probably absent:
Based on the solubility properties of the Group I cations, we know that silver chloride (AgCl) is sparingly soluble in water, lead chloride (PbCl2) is fairly soluble, and mercury(I) chloride (Hg2Cl2) is moderately soluble. If the majority of the precipitate dissolves in hot water, it suggests that both lead chloride and mercury(I) chloride would have dissolved since they are soluble to some extent. Therefore, the ions that are most likely absent are lead (Pb2+) and mercury(I) (Hg2+) ions.
In summary, based on the solubility behavior after precipitation with HCl and subsequent dissolution in hot water, the chloride ion (Cl-) is most likely present, while the lead (Pb2+) and mercury(I) (Hg2+) ions are most likely absent.