Question

Suppose you have a solution that might contain any or all of the following cations: \(\mathrm{Ni}^{2+}, \mathrm{Ag}^{+}, \mathrm{Sr}^{2+},\) and \(\mathrm{Mn}^{2+}\). Addition of \(\mathrm{HCl}\) solution causes a precipitate to form. After filtering off the precipitate, \(\mathrm{H}_{2} \mathrm{SO}_{4}\) solution is added to the resulting solution and another precipitate forms. This is filtered off, and a solution of \(\mathrm{NaOH}\) is added to the resulting solution. No precipitate is observed. Which ions are present in each of the precipitates? Which of the four ions listed above must be absent from the original solution?

Short answer

The ions present in precipitates are Ag+ and Sr2+. Ni2+ and Mn2+ must be absent.

Step-by-step solution

Identify the Ion that Forms Precipitate with HCl

When HCl is added to the solution, it can potentially form a precipitate with silver ion, \(\text{Ag}^+\), because \(\text{AgCl}\) is a well-known insoluble chloride. None of the other ions (\(\text{Ni}^{2+}, \text{Sr}^{2+},\) and \(\text{Mn}^{2+}\)) form insoluble chlorides.Therefore, the first precipitate that forms is likely \(\text{AgCl}\), indicating the presence of \(\text{Ag}^+\).

Determine Ions that Form Precipitate with H2SO4

After filtering off the \(\text{AgCl}\) precipitate, the solution is treated with \(\text{H}_{2}\text{SO}_{4}\). Among the ions present, \(\text{Sr}^{2+}\) will form an insoluble sulfate, \(\text{SrSO}_{4}\), because it is slightly soluble. \(\text{Ni}^{2+}\) and \(\text{Mn}^{2+}\) do not form insoluble sulfates.Thus, the second precipitate is \(\text{SrSO}_{4}\), indicating the presence of \(\text{Sr}^{2+}\).

Assess Formation of Precipitate with NaOH

Once the \(\text{SrSO}_{4}\) is filtered off, NaOH is added to the remaining solution. If \(\text{Ni}^{2+}\) or \(\text{Mn}^{2+}\) were present, they would form insoluble hydroxides, \(\text{Ni(OH)}_{2}\) and \(\text{Mn(OH)}_{2}\), respectively. Since no precipitate forms, neither \(\text{Ni}^{2+}\) nor \(\text{Mn}^{2+}\) are present in solution.Therefore, both \(\text{Ni}^{2+}\) and \(\text{Mn}^{2+}\) ions must be absent from the original solution.

Key concepts

Cations

Cations are positively charged ions, and they play a crucial role in the qualitative analysis of solutions. In the context of ion precipitation, being able to recognize the behavior of cations can help identify substances in a mixture. When a solution is known to contain potential cations like \(\mathrm{Ni}^{2+}\), \(\mathrm{Ag}^{+}\), \(\mathrm{Sr}^{2+}\), and \(\mathrm{Mn}^{2+}\), understanding the unique attributes of each cation becomes essential. For example, \(\mathrm{Ag}^{+}\) is notorious for forming an insoluble white precipitate with chloride ions, \(\mathrm{Cl}^{-}\), resulting in \(\mathrm{AgCl}\). This distinct reaction is invaluable in detecting the presence of silver ions in a solution.
Overall, grasping the properties and reactions of cations allows for precise determination of ion presence, especially when different reagents are introduced to the solution sequentially. This step-by-step identification through precipitation further underscores the importance of cation analysis in chemistry.

Precipitation Reactions

Precipitation reactions involve the formation of an insoluble compound from the mixture of two soluble salts, resulting in a solid. These reactions are key in distinguishing different ions that may be present in a solution, as each ion will typically produce distinct precipitates when reacting with particular reagents.Consider the scenario where hydrochloric acid (HCl) is added to a solution. - We expect to observe a precipitate if silver ions, \(\mathrm{Ag}^{+}\), are present, as they react to form the solid \(\mathrm{AgCl}\).- This phenomenon doesn't happen with \(\mathrm{Ni}^{2+}\), \(\mathrm{Sr}^{2+}\), or \(\mathrm{Mn}^{2+}\), making the presence of the silver ion obvious right away.Next, when \(\mathrm{H}_{2}\mathrm{SO}_{4}\) is added after filtering, the formation of \(\mathrm{SrSO}_{4}\) can reveal the presence of \(\mathrm{Sr}^{2+}\), as it forms an insoluble sulfate. By understanding and predicting these reactions, it's possible to isolate and identify ions in a mixed solution effectively.

Chemical Solubility Rules

Chemical solubility rules are essential guidelines that predict the outcome of precipitation reactions. These rules help us anticipate whether a salt will dissolve in water or form a solid precipitate. By mastering these rules, we can deduce the behavior of ions in different chemical environments.The solubility rules state:- Most chloride salts, like NaCl, are soluble. However, silver chloride (\(\mathrm{AgCl}\)) is an exception and is insoluble in water.- Sulfate salts are typically soluble, yet compounds such as \(\mathrm{SrSO}_{4}\) are only slightly soluble, leading to precipitation in solutions with \(\mathrm{Sr}^{2+}\).- Hydroxides are usually insoluble, but since no precipitate formed with \(\mathrm{NaOH}\), we conclude that cations like \(\mathrm{Ni}^{2+}\) and \(\mathrm{Mn}^{2+}\) were not present.These rules are crucial for the qualitative analysis of chemical mixtures, where determining which ions form precipitates under specific conditions allows chemists to identify unknown cations efficiently.