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 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 first precipitate (\(\mathrm{AgCl}\)) indicates the presence of \(\mathrm{Ag}^{+}\) ions, and the second precipitate (\(\mathrm{SrSO}_{4}\)) indicates the presence of \(\mathrm{Sr}^{2+}\) ions in the original solution. Both \(\mathrm{Ni}^{2+}\) and \(\mathrm{Mn}^{2+}\) ions are absent from the original solution.

Step-by-step solution

Identify the possible precipitates with HCl

When HCl is added to the solution containing the cations, it reacts with them and possibly forms precipitates if the compounds are sparingly soluble. Check for the solubility of the resulting halides in each case: 1. \(\mathrm{Ni}^{2+}\) forms \(\mathrm{NiCl}_{2}\) which is soluble in water. 2. \(\mathrm{Ag}^{+}\) forms \(\mathrm{AgCl}\) which is insoluble in water. 3. \(\mathrm{Sr}^{2+}\) forms \(\mathrm{SrCl}_{2}\) which is soluble in water. 4. \(\mathrm{Mn}^{2+}\) forms \(\mathrm{MnCl}_{2}\) which is soluble in water. The first precipitate is \(\mathrm{AgCl}\) which indicates the presence of \(\mathrm{Ag}^{+}\) ions.

Identify the possible precipitates with H2SO4

When H2SO4 is added to the solution containing the remaining cations, it reacts with them and possibly forms precipitates if the compounds are sparingly soluble. Check the solubility of the resulting sulfates in each case: 1. \(\mathrm{Ni}^{2+}\) with \(\mathrm{H}_{2} \mathrm{SO}_{4}\) forms \(\mathrm{NiSO}_{4}\) which is soluble in water. 2. \(\mathrm{Sr}^{2+}\) with \(\mathrm{H}_{2} \mathrm{SO}_{4}\) forms \(\mathrm{SrSO}_{4}\) which is sparingly soluble in water. 3. \(\mathrm{Mn}^{2+}\) with \(\mathrm{H}_{2} \mathrm{SO}_{4}\) forms \(\mathrm{MnSO}_{4}\) which is soluble in water. The second precipitate is \(\mathrm{SrSO}_{4}\) which indicates the presence of \(\mathrm{Sr}^{2+}\) ions.

Check for precipitate formation with NaOH

Finally, NaOH is added to the solution containing the remaining cations, and no precipitate is observed. We need to check for the formation of any insoluble hydroxides with the remaining cations: 1. \(\mathrm{Ni}^{2+}\) forms \(\mathrm{Ni(OH)}_{2}\) which is sparingly soluble in water. 2. \(\mathrm{Mn}^{2+}\) forms \(\mathrm{Mn(OH)}_{2}\) which is sparingly soluble in water. Since no precipitate is observed after adding NaOH, it means that neither \(\mathrm{Ni}^{2+}\) nor \(\mathrm{Mn}^{2+}\) ions were present in the original solution.

Final answer

The first precipitate (\(\mathrm{AgCl}\)) indicates the presence of \(\mathrm{Ag}^{+}\) ions, and the second precipitate (\(\mathrm{SrSO}_{4}\)) indicates the presence of \(\mathrm{Sr}^{2+}\) ions in the original solution. Both \(\mathrm{Ni}^{2+}\) and \(\mathrm{Mn}^{2+}\) ions are absent from the original solution.

Key concepts

Cation Precipitation

Cation precipitation is a key concept in qualitative analysis. When a solution containing various cations is treated with specific reagents, some of these cations form solid precipitates. This process helps in identifying which ions are present in a solution.

In the exercise, when hydrochloric acid (HCl) is added to the solution, it reacts with cations to form chlorides. We know that some chlorides, like \(\mathrm{AgCl}\), do not dissolve in water easily and will precipitate out of the solution. AgCl's insolubility in water allows us to confidently say that the presence of a white precipitate indicates \(\mathrm{Ag}^{+}\) ions.

Later, when \(\mathrm{H}_{2}\mathrm{SO}_{4}\) is added, it can form sulfates with other cations that might also precipitate if insoluble. In this exercise, \(\mathrm{SrSO}_{4}\) is the precipitate formed, showing the presence of \(\mathrm{Sr}^{2+}\) ions. Using these precipitation reactions effectively separates cations based on their solubility characteristics.

Solubility Rules

Solubility rules are guidelines that help predict whether a compound will dissolve in water. Understanding these rules is vital for predicting the outcome of reactions in aqueous solutions.

For instance, most chloride salts like \(\mathrm{NaCl}\) are soluble in water, but \(\mathrm{AgCl}\) is an exception; it is insoluble. This makes \(insolubility\) a useful property for identifying \(Ag⁺\) ions through precipitate formation when mixed with a chloride source, like \(HCl\).

Another key example involves sulfates: while most are soluble, \(\mathrm{SrSO}_{4}\) is not. This selective insolubility is crucial for pinpointing \(Sr²⁺\) ions when \(H_{2}SO_{4}\) is added. Mastering these solubility rules allows you to predict which ions will precipitate under different conditions, thus aiding in their identification.

Chemical Reactions in Aqueous Solutions

Chemical reactions in aqueous solutions involve substances that dissolve in water to undergo reactions. These reactions can lead to the formation of precipitates, gas release, or even color changes, helping chemists identify unknown substances.

In our exercise, the addition of \(HCl\) initiates a reaction with dissolved cations. The formation of a solid precipitate such as \(\mathrm{AgCl}\) is a sign of a chemical reaction where \(Ag⁺\) ions are removed from the solution.

After filtering, \(H_{2}SO_{4}\) reacts with the remaining ions, leading to the formation of \(\mathrm{SrSO}_{4}\) when \(Sr²⁺\) is present. This shows how sequential chemical reactions can be used to identify components within a mixed solution. Understanding these reactions is crucial for analyzing and interpreting results accurately in a laboratory setting.