Question

Separate samples of a solution of an unknown ionic compound are treated with dilute \(\mathrm{AgNO}_{3}, \mathrm{~Pb}\left(\mathrm{NO}_{3}\right)_{2}\), and \(\mathrm{BaCl}_{2}\). Precipitates form in all three cases. Which of the following could be the anion of the unknown salt: \(\mathrm{Br}^{-}, \mathrm{CO}_{3}{ }^{2-}, \mathrm{NO}_{3}{ }^{-}\)?

Short answer

The anion of the unknown salt must form precipitates with AgNO3, Pb(NO3)2, and BaCl2. Based on our analysis, Br- and CO3^2- can form precipitates with these compounds, while NO3- cannot. Thus, the anion of the unknown salt could be either Br- or CO3^2-.

Step-by-step solution

Analyze the reactivity of Br-

In the presence of AgNO3, Br- would react as follows: AgNO3(aq) + XBr(aq) → AgBr(s) + XNO3(aq) AgBr is almost insoluble and would precipitate. Br- would also react with Pb(NO3)2 and BaCl2 in similar ways: Pb(NO3)2(aq) + 2XBr(aq) → PbBr2(s) + 2XNO3(aq) BaCl2(aq) + 2XBr(aq) → BaBr2(s) + 2XCl-(aq) Both PbBr2 and BaBr2 are slightly soluble in water, allowing the formation of precipitates. So, based on these reactions, Br- could possibly be the anion of the unknown salt.

Analyze the reactivity of CO3^2-

In the presence of AgNO3, CO3^2- would react as follows: 2AgNO3(aq) + XCO3^2-(aq) → Ag2CO3(s) + 2XNO3(aq) Ag2CO3 is almost insoluble and would precipitate. CO3^2- would also react with Pb(NO3)2 and BaCl2 in similar ways: Pb(NO3)2(aq) + XCO3^2-(aq) → PbCO3(s) + 2XNO3(aq) BaCl2(aq) + XCO3^2-(aq) → BaCO3(s) + 2XCl-(aq) Both PbCO3 and BaCO3 are also almost insoluble in water, allowing the formation of precipitates. So, based on these reactions, CO3^2- could also possibly be the anion of the unknown salt.

Analyze the reactivity of NO3-

In the presence of AgNO3, NO3- would react as follows: AgNO3(aq) + XNO3(aq) → AgNO3(aq) + XNO3(aq) Since AgNO3 is soluble in water, there wouldn't be any precipitate formed with NO3-. The same will happen with reactions of NO3- with Pb(NO3)2 and BaCl2. So, NO3- cannot be the anion of the unknown salt as it doesn't form precipitates.

Conclusion

The anion of the unknown salt must form precipitates with AgNO3, Pb(NO3)2, and BaCl2. Based on our analysis, Br- and CO3^2- can form precipitates with these compounds, while NO3- cannot. Thus, the anion of the unknown salt could be either Br- or CO3^2-.

Key concepts

Precipitation Reactions

Precipitation reactions occur when two soluble salts react in aqueous solution to form an insoluble solid, called a precipitate. When you mix solutions containing ionic compounds, they dissociate into individual ions in water. If a combination of these ions forms an insoluble compound, a precipitate forms, visually signaling the reaction.

In the given exercise, unknown ions react with

• AgNO₃ to form AgBr or Ag₂CO₃
• Pb(NO₃)₂ to form PbBr₂ or PbCO₃
• BaCl₂ to form BaBr₂ or BaCO₃

These reactions result in a precipitate because the products are not soluble in water. Identifying which precipitates form helps determine the anion in the unknown ionic compound.

Anion Identification

Identifying an anion involves testing reactions that can distinctly reveal the presence of specific ions. In this exercise, the goal is to determine which anion forms precipitates with silver, lead, and barium compounds.

By forming a precipitate with

• AgNO₃
• Pb(NO₃)₂
• BaCl₂

Bromide (Br⁻) and carbonate (CO₃²⁻) ions indicate their presence. Since these ions create insoluble compounds, observing precipitates cues us to their identity. In contrast, nitrate (NO₃⁻) does not form a precipitate, helping rule out its presence in the tested compound.

Hence, testing which reactions result in precipitation provides the necessary clues for identifying the unknown anion.

Chemical Solubility

Understanding chemical solubility is key to predicting which compounds will form precipitates. Solubility rules tell us which ionic compounds dissolve in water and which do not. Solubility is influenced by factors like lattice energy of the solid and the hydration energy of the ions.

For example, most nitrate ( NO₃⁻) compounds are soluble, so they don't typically form precipitates. Bromides ( Br⁻), however, are only slightly soluble with lead and silver, and most carbonates (like CO₃²⁻) are insoluble, forming a solid in solution when mixed with ions like Ag⁺, Pb²⁺, or Ba²⁺.

This knowledge allows chemists to predict precipitation reactions and identify unknown ions by observing results in a controlled setting.