Question

A solution containing several metal ions is treated with dilute HCl; no precipitate forms. The pH is adjusted to about 1, and \(\mathrm{H}_{2} \mathrm{~S}\) is bubbled through. Again, no precipitate forms. The \(\mathrm{pH}\) of the solution is then adjusted to about 8 . Again, \(\mathrm{H}_{2} \mathrm{~S}\) is bubbled through. This time a precipitate forms. The filtrate from this solution is treated with \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{HPO}_{4}\). No precipitate forms. Which of these metal cations are either possibly present or definitely absent: \(\mathrm{Al}^{3+}, \mathrm{Na}^{+}, \mathrm{Ag}^{+}, \mathrm{Mg}^{2+}\) ?

Short answer

Based on the given exercise, Al³⁺ and Ag⁺ are definitely absent, while Na⁺ and Mg²⁺ are possibly present. No precipitate was formed in the presence of HCl and (NH₄)₂HPO₄ for Al³⁺, and no precipitate formed after treating with dilute HCl for Ag⁺. On the other hand, Na⁺ could be present as NaCl is soluble in dilute HCl, Na₂S is soluble at pH 8, and no precipitate was observed after adding (NH₄)₂HPO₄. Similarly, Mg²⁺ could be present since MgCl₂ is soluble in dilute HCl, MgS is insoluble at pH 8, and no precipitate was observed after adding (NH₄)₂HPO₄.

Step-by-step solution

Effects of dilute HCl on metal ions

The first thing is to understand what happens when we treat a solution containing metal ions with dilute HCl. Chlorides of certain metal ions are insoluble and will precipitate when HCl is added. However, no precipitate is formed in this case, which means that Al³⁺ and Ag⁺ ions are not present because AlCl₃ and AgCl are soluble in dilute HCl.

Effects of H₂S at pH 1 on metal ions

At a low pH (around 1), H₂S will react with metal ions to form metal sulfides. Since there is no precipitate formation, we can conclude that none of the metal sulfides are formed. Therefore, at this stage, we can't make any further conclusion about the presence or absence of Na⁺ or Mg²⁺ ions.

Effects of H₂S at pH 8 on metal ions

When we adjust the pH to about 8 and pass H₂S through the solution again, a precipitate is formed. This means that there are metal ions present that can form insoluble metal sulfides at this pH. MgS is insoluble at this pH, so Mg²⁺ ions could possibly be present. On the other hand, Na₂S is soluble at this pH which means Na⁺ ions are not forming a precipitate, but we still cannot confirm their absence.

Effects of (NH₄)₂HPO₄ on metal ions

Finally, after filtering the solution from step 3, (NH₄)₂HPO₄ is added and no precipitate is formed. This indicates that there are no metal ions present that can form insoluble metal phosphates. In our list of ions, Al³⁺ can form insoluble AlPO₄, but since no precipitate is observed, we can conclude that Al³⁺ ions are definitely absent. #Conclusion# In summary, based on the given exercise: - Al³⁺ is definitely absent, because no precipitate was formed in the presence of HCl and (NH₄)₂HPO₄. - Na⁺ is possibly present, as NaCl is soluble in dilute HCl, Na₂S is soluble at pH 8, and no precipitate was observed after adding (NH₄)₂HPO₄. - Ag⁺ is definitely absent, due to no precipitate formation after treating the solution with dilute HCl. - Mg²⁺ is possibly present, as MgCl₂ is soluble in dilute HCl, MgS is insoluble at pH 8, and no precipitate was observed after adding (NH₄)₂HPO₄.

Key concepts

Dilute HCl Reactions

Dilute hydrochloric acid (HCl) is often used to test for the presence of certain metal ions based on their ability to form insoluble chlorides. When a solution containing multiple metal ions is treated with dilute HCl and no precipitate forms, it typically means that the metal ions present do not form insoluble chlorides.
- For instance, aluminum chloride (AlCl₃) and sodium chloride (NaCl) are both soluble in water, which means that if these ions are present in the solution, they will not precipitate out with the addition of dilute HCl. - Silver chloride (AgCl), however, is not soluble in water and would form a precipitate if silver ions were present. The absence of such a precipitate indicates the absence of Ag⁺ ions.
Using dilute HCl as a reagent relies on the solubility product principle, which helps predict whether a precipitate will form based on the concentrations of its constituent ions. Thus, observing no precipitate when adding dilute HCl suggests the absence of metal ions that form insoluble chlorides under the conditions tested.

pH Adjustment Effects

Adjusting the pH of a solution can dramatically change the solubility of metal ions, which affects whether or not they will precipitate as insoluble compounds. At pH 1, the solution is quite acidic, and only very few metal sulfides will precipitate due to the low concentration of sulfide ions.
This is why there was no precipitate with H₂S at pH 1 in the given exercise. However, when the pH is adjusted to about 8, which is weakly alkaline, the concentration of sulfide ions in solution increases, allowing metal ions like Mg²⁺, which form insoluble metal sulfides, to precipitate.

• This change illustrates the principle that pH modification can either suppress or enhance precipitation reactions.
• In a basic medium, as indicated by pH 8, more sulfide ions are available to react with metal cations to form a solid.

Therefore, understanding the solubility of compounds at different pH levels is crucial to controlling chemical reactions and predicting outcomes accurately.

Metal Sulfides

Metal sulfides are compounds formed when metal cations react with sulfide ions. The solubility of these sulfides can vary significantly with pH.
- At high acidity (low pH), the sulfide ion concentration is low, which tends to prevent precipitation. - As the pH approaches neutral to alkaline levels, more sulfide ions are present, and various metal ions can form insoluble sulfide precipitates.
In the context of the exercise, Mg²⁺ potentially formed magnesium sulfide (MgS) at pH 8, illustrating that certain metal cations react more readily at specific pH levels to form insoluble products. This property is leveraged in qualitative chemical analysis to identify the presence of different metal ions.

• Metals able to form precipitates at higher pH values can often be separated from those that remain soluble, allowing for differentiated detection.

Adjusting the pH to encourage specific metal sulfide precipitation is a critical technique in analytical chemistry.

Metal Phosphates

Metal phosphates like aluminum phosphate (AlPO₄) are another category of compounds that can form under certain conditions. In this exercise, (NH₄)₂HPO₄ was added to determine the presence of ions capable of forming metal phosphates.
AlPO₄ is known to be insoluble, so if aluminum ions were present, we would expect to see a precipitate form when the solution is treated with (NH₄)₂HPO₄. The absence of a precipitate indicates that Al³⁺ ions are not in the solution.

• This step is crucial for concluding the absence of Al³⁺, as the methodology relies on the solubility characteristics of phosphates similar to that of sulfides and chlorides in earlier steps.

Using different reagents to test for solubility helps paint a comprehensive picture of the ionic composition of a solution, facilitating the determination of specific ions that are present or absent.