Problem 76 - General Chemistry: Principles and Modern Applications

Question

An unknown white solid consists of two compounds, each containing a different cation. As suggested in the illustration, the unknown is partially soluble in water. The solution is treated with \(\mathrm{NaOH}(\mathrm{aq})\) and yields a white precipitate. The part of the original solid that is insoluble in water dissolves in \(\mathrm{HCl}(\mathrm{aq})\) with the evolution of a gas. The resulting solution is then treated with \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}(\mathrm{aq})\) and yields a white precipitate. (a) Is it possible that any of the cations \(M g^{2+}, C u^{2+}\) \(\mathrm{Ba}^{2+}, \mathrm{Na}^{+},\) or \(\mathrm{NH}_{4}^{+}\) were present in the original unknown? Explain your reasoning. (b) What compounds could be in the unknown mixture (that is, what anions might be present)?

Step-by-Step Solution

Answer

The original unknown solid could contain the cation \(\mathrm{Ba}^{2+}\) but not \(M g^{2+}, C u^{2+}, \mathrm{Na}^{+}\), or \(\mathrm{NH}_{4}^{+}\). The possible compounds in the unknown mixture could be \(\mathrm{BaCO}_3\) (which would evolve a gas when reacted with \(\mathrm{HCl}\)) and \(\mathrm{BaSO}_4\) (which would form a white precipitate when reacted with \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}(\mathrm{aq})\)).

Step 1: Evaluate possible cations

The problem describes that a part of the unknown solid mixture forms a white precipitate after adding \(\mathrm{NaOH}(\mathrm{aq})\). This suggests the presence of cations that could form a insoluble hydroxide when react with NaOH. This would rule out \(\mathrm{Na}^{+}\) and \(\mathrm{NH}_{4}^{+}\), since their hydroxides (\(\mathrm{NaOH}\) and \(\mathrm{NH}_4\mathrm{OH}\)) are soluble in water. The remaining cations to consider are \(M g^{2+}, C u^{2+}\), and \(\mathrm{Ba}^{2+}\)

Step 2: Identify cations that react with \(\mathrm{HCl}(\mathrm{aq})\)

The problem also describes that the part of the original solid that is insoluble in water could dissolve in \(\mathrm{HCl}(\mathrm{aq})\) with the evolution of a gas. The only cation from those remaining that could undergo such reaction is \(\mathrm{Ba}^{2+}\), since \(\mathrm{BaCO}_3\) is insoluble in water, soluble in \(\mathrm{HCl}\), and produces \(\mathrm{CO}_2\) gas. Therefore, the original compound could contain \(\mathrm{Ba}^{2+}\) cation

Step 3: Evaluate possible anions

The problem mentions that the solution obtained after treating with \(\mathrm{HCl}(\mathrm{aq})\) yields a white precipitate when treated with \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}(\mathrm{aq})\). This reaction indicates the presence of a cation that can form an insoluble sulphate. This again points toward \(\mathrm{Ba}^{2+}\) as \(\mathrm{BaSO}_4\) is insoluble. Therefore, one of the anions in the original compound could be \(\mathrm{SO}_4^{2-}\)

Step 4: Identify the original unknown mixture

From steps 2 and 3, the unknown solid could possibly be a mixture of two compounds containing \(\mathrm{Ba}^{2+}\) cation and one of these compounds might contain \(\mathrm{SO}_4^{2-}\) anion. A possible compound consistent with the solubility properties could be \(\mathrm{BaCO}_3\), which dissolves in \(\mathrm{HCl}\) with the evolution of gas.

Key Concepts

Solubility Rules

In chemistry, solubility rules help predict whether a compound will dissolve in water. These guidelines are crucial when trying to identify unknown compounds in a mixture. Generally,

• Most salts containing alkali metals like sodium (Na⁺) and ammonium (NH₄⁺) are soluble.
• Nitrates ( O_3^{-} ) and acetates ( C_2H_3O_2^{−} ) also dissolve in water.
• However, carbonates ( CO_3^{2-} ), phosphates ( PO_4^{3-} ), sulfides ( S^{2-} ), and hydroxides ( OH^{-} ) are typically insoluble except for those of the alkali metals and a few exceptions.

In the original exercise, these rules are used to rule out some cations based on potential solubility outcomes with added solutions like NaOH and hydrochloric acid (HCl). Understanding these basics helps determine which substances will form a precipitate in certain reactions.

Precipitation Reactions

Precipitation reactions occur when two solutions mix, resulting in the formation of an insoluble product known as a precipitate. This process is often used in cation analysis to identify specific ions in a mixture. When a white precipitate forms after adding sodium hydroxide ( ext{NaOH} ), it suggests the creation of an insoluble hydroxide compound. Common cations such as Mg^{2+} , Cu^{2+} , and Ba^{2+} can form such hydroxides. The white solid helps narrow the possibilities for the cations present in the unknown mixture. Recognizing these reactions is crucial for determining whether the components in a sample will remain dissolved or become visible as a solid.

Insoluble Compounds

Certain compounds remain insoluble in water, meaning they cannot dissolve, making them key indicators in chemical identification processes. For instance, compounds like BaCO_3 (barium carbonate) are typically insoluble. In the context of the exercise, the insoluble part of the original solid that dissolves in HCl is likely a carbonate, due to the release of gas during the reaction. Insolubility not only assists in identifying chemical substances but also in understanding their interactions when combining solutions in a lab setting. This knowledge aids in predicting when a visible reaction might occur.

Chemical Reactions with Acids

Chemical reactions with acids often result in noticeable changes such as gas evolution or color change, which are clues in detecting specific compounds. For example, when an insoluble carbonate such as BaCO_3 reacts with HCl , carbon dioxide gas ( CO_2 ) is released as seen in the original exercise. Moreover, these reactions can lead to the dissolution of some solids otherwise insoluble in water, revealing further reaction pathways and potential ions present. Being able to predict and observe these reactions can help determine the composition of unknown mixtures and is essential for cation analysis.