Question

Separate samples of a solution of an unknown soluble ionic compound are treated with \(\mathrm{KCl}, \mathrm{Na}_{2} \mathrm{SO}_{4},\) and \(\mathrm{NaOH}\) . A precipitate forms only when \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) is added. Which cations could be present in the unknown soluble ionic compound?

Short answer

The cations that could be present in the unknown soluble ionic compound are \(Ba^{2+}\) (barium), \(Pb^{2+}\) (lead), and \(Sr^{2+}\) (strontium), based on the reactions with \(Na_2SO_4\), \(KCl\), and \(NaOH\).

Step-by-step solution

Analyze the reaction with KCl

We're given that no precipitate forms when KCl is added, which means none of the cations in the unknown ionic compound form an insoluble compound with Cl-. Therefore, the cation present in the unknown compound cannot create an insoluble chloride.

Analyze the reaction with Na2SO4

A precipitate is formed when Na2SO4 is added. So, our cation must form an insoluble compound with SO4^2-. We can list some possible cations that form insoluble sulfates, such as: - Ba^2+ (barium) - Pb^2+ (lead) - Sr^2+ (strontium) - Ag^+ (silver)

Analyze the reaction with NaOH

There is no reaction when NaOH is added to the unknown compound. Therefore, the cation in the unknown compound will not form an insoluble compound with OH- ions. This fact eliminates Ag^+ from the possibilities, since silver forms an insoluble hydroxide.

Conclusion

Based on the given information, the cations that could be present in the unknown soluble ionic compound are Ba^2+ (barium), Pb^2+ (lead), and Sr^2+ (strontium).

Key concepts

Ionic Compounds

Ionic compounds are fascinating substances made up of positively charged ions called cations and negatively charged ions known as anions. These ions come together to form a stable compound, balanced in charge. For example, table salt (\(\text{NaCl}\)) is an ionic compound made of sodium (\(\text{Na}^+\)) and chloride (\(\text{Cl}^-\)) ions.

• Ionic compounds tend to have high melting and boiling points because of the strong attraction between the ions.
• They often dissolve in water, becoming ions again.
• In water, ionic compounds can conduct electricity due to the movement of their ions.

Understanding the behavior of ionic compounds is essential, especially when observing how they interact in solution. They might react with other ions to form new compounds, some of which can be insoluble. This ability leads to various applications and reactions like precipitation.
Recognizing cations and their tendencies to form insoluble compounds is crucial in many chemical processes.

Precipitation Reactions

Precipitation reactions occur when two solutions containing dissolved ions are mixed, and an insoluble solid, known as a precipitate, forms. These reactions are important in chemistry for identifying ions and understanding solution behavior.

• A precipitate forms when the product of the reaction is not soluble in the solvent, commonly water.
• For example, when barium chloride reacts with sulfuric acid, barium sulfate precipitates because it is insoluble in water.

The golden rule to predict whether a precipitate will form is to refer to solubility rules. These rules help determine which combinations of ions will form insoluble compounds, making it easier to predict the result of a reaction.
In our exercise, the addition of \(\text{Na}_2\text{SO}_4\) led to the precipitation of the cation, revealing the presence of specific elements that form insoluble sulfates. Such insights are invaluable in chemical analysis and various industrial applications.

Cation Identification

Identifying cations in a solution involves observing the reactions they undergo with various reagents. In our exercise, the unknown ionic compound's behavior with certain additives helped pin down potential cations.

• When treated with \(\text{KCl}\), no precipitate formed. This ruled out cations that form insoluble chlorides, like \(\text{Ag}^+\).
• Adding \(\text{Na}_2\text{SO}_4\) triggered a precipitate, indicating cations like \(\text{Ba}^{2+}\), \(\text{Pb}^{2+}\), and \(\text{Sr}^{2+}\), known for forming insoluble sulfates.
• With no reaction upon addition of \(\text{NaOH}\), cations such as \(\text{Ag}^+\) were further ruled out, as they form insoluble hydroxides.

Using these observations, cations such as barium, lead, and strontium were identified as possibilities. This methodical approach highlights the power of selective precipitation in analytical chemistry, offering a pathway to decipher complex ionic mixtures.