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 barium (Ba²⁺) and strontium (Sr²⁺), as they form a precipitate with SO₄²⁻ but not with Cl⁻ or OH⁻.

Step-by-step solution

Identify possible precipitation reactions

When KCl, Na2SO4, and NaOH are added to the solution, the ions may react with the anions/cations to form precipitates. The precipitation reactions can be defined as: 1. KCl added: K⁺ + unknown anion → precipitate 2. Na2SO4 added: 2Na⁺ + unknown cation → precipitate 3. NaOH added: Na⁺ + unknown anion → precipitate

Analyze possible cations in the given scenarios

In scenario 1 and 3, no precipitate forms, meaning the unknown anion doesn't react with K⁺ or OH⁻ ions. In scenario 2, the formation of a precipitate indicates that the unknown cation reacts with SO₄²⁻ ions. The common cations that react with SO₄²⁻ ions to form precipitates are Ba²⁺, Sr²⁺, Pb²⁺, and Ca²⁺. Thus, the unknown soluble ionic compound must have a cation that forms a precipitate when SO₄²⁻ is added but not when Cl⁻ or OH⁻ is added. Let's analyze the possible precipitation reactions for each cation.

Check the solubility rules for the possible cations

Ba²⁺ - BaCl₂: soluble (Ba²⁺ has no precipitate with Cl⁻) - BaSO₄: insoluble (formation of precipitate) - Ba(OH)₂: soluble (Ba²⁺ has no precipitate with OH⁻) Sr²⁺ - SrCl₂: soluble (Sr²⁺ has no precipitate with Cl⁻) - SrSO₄: insoluble (formation of precipitate) - Sr(OH)₂: soluble (Sr²⁺ has no precipitate with OH⁻) Pb²⁺ - PbCl₂: insoluble (Pb²⁺ forms a precipitate with Cl⁻) - PbSO₄: insoluble (formation of precipitate) - Pb(OH)₂: insoluble (Pb²⁺ forms a precipitate with OH⁻) Ca²⁺ - CaCl₂: soluble (Ca²⁺ has no precipitate with Cl⁻) - CaSO₄: slightly insoluble (formation of precipitate) - Ca(OH)₂: insoluble (Ca²⁺ forms a precipitate with OH⁻)

Match the cations with the observed reactions

Observations: 1. No precipitate with KCl (no precipitate with Cl⁻) 2. Precipitate with Na2SO4 (precipitate with SO₄²⁻) 3. No precipitate with NaOH (no precipitate with OH⁻) From the solubility rules analyzed in step 3, the cations that match the observations are Ba²⁺ and Sr²⁺. Ba²⁺ 1. No precipitate with Cl⁻ 2. Precipitate with SO₄²⁻ 3. No precipitate with OH⁻ Sr²⁺ 1. No precipitate with Cl⁻ 2. Precipitate with SO₄²⁻ 3. No precipitate with OH⁻

Conclusion

The cations that could be present in the unknown soluble ionic compound are barium (Ba²⁺) and strontium (Sr²⁺).

Key concepts

Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between positively charged ions, known as cations, and negatively charged ions, called anions. These bonds occur between metals and non-metals. Upon dissolving in water, ionic compounds dissociate into their constituent ions. For instance, a compound like sodium chloride (NaCl) will separate into sodium ions (Na⁺) and chloride ions (Cl⁻) when dissolved.
Recognizing ionic compounds is crucial because their behavior in solution is fundamental to understanding reactions like precipitation. Ionic compounds have distinct properties, such as high melting and boiling points, and they often conduct electricity when dissolved in water. This conductive property is key to many chemical processes and analyses, such as cation identification in solutions.
When working with ionic compounds, knowledge of their dissociation can help predict outcomes of reactions, like whether a precipitate will form or what ions are present in solution. This is particularly beneficial when dealing with unknown solutions, as it aids in identifying potential ions through reactions.

Precipitation Reactions

Precipitation reactions involve the formation of an insoluble solid, or precipitate, when solutions containing soluble salts are mixed. These reactions are a subset of double replacement reactions, where ions switch partners to form new compounds. A classic example is the reaction between barium chloride (BaCl₂) and sodium sulfate (Na₂SO₄), which results in the formation of barium sulfate (BaSO₄) as a precipitate.
In the context of identifying unknown ionic compounds, precipitation reactions are a useful tool. By adding specific reagents to a solution, we can determine which ions are present based on whether a precipitate forms. This requires knowing which combinations of ions result in insoluble compounds.

• A precipitate with Na₂SO₄ suggests the possible presence of cations like Ba²⁺ or Sr²⁺ that form insoluble sulfates.
• No precipitate with Cl⁻ or OH⁻ further narrows down the possibilities.

By monitoring which mixtures form precipitates, chemists can use these reactions strategically to analyze and identify unknown solutions.

Solubility Rules

Solubility rules are essential guidelines that help predict the solubility of ionic compounds in water. These rules aren't hard and fast, but they offer a good approximation for most circumstances. For example:

• Sulfates (SO₄²⁻) are generally soluble, with exceptions like BaSO₄, SrSO₄, and PbSO₄, which are insoluble.
• Chlorides (Cl⁻) are typically soluble, but not when paired with Ag⁺, Pb²⁺, and Hg₂²⁺.
• Hydroxides (OH⁻) tend to be insoluble, except for those of alkali metals and some alkaline earth metals like Ba²⁺ and Sr²⁺.

Understanding these rules allows us to predict whether a given ionic compound will dissolve or form a precipitate. For instance, the formation of a precipitate when Na₂SO₄ is added indicates the presence of cations like Ba²⁺ or Sr²⁺.
These rules are fundamental to performing accurate cation analysis and solving chemical problems involving solubility and precipitation. They aid in narrowing down potential ions in a solution by showing how they interact with known anions.

Cation Analysis

Cation analysis involves identifying which cations are present in a sample through the use of various chemical reactions, including precipitation reactions. This technique is crucial in areas such as quality control, pollution monitoring, and laboratory analysis.
In the given exercise, the absence or presence of a precipitate provides clues about the cations involved:

• No precipitate with KCl or NaOH rules out cations that form insoluble compounds with Cl⁻ or OH⁻.
• A precipitate with Na₂SO₄ suggests a reaction with SO₄²⁻, indicating cations like Ba²⁺ or Sr²⁺ could be present.

By understanding the solubility behavior and reaction tendencies of cations, such as their proclivity to react with sulfate ions to form a precipitate, chemists can systematically identify the ions in a solution. Cation analysis often utilizes a sequence of specific tests, each contributing to a more conclusive identification of unknown cations. This methodical approach enables accurate and reliable determination of ionic compositions in various chemical samples.