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 is CO3^2- (carbonate ion).

Step-by-step solution

Understand the reaction with AgNO3

When silver nitrate (AgNO3) is added to a solution that contains anions, it forms precipitates with certain anions. Silver bromide (AgBr) and silver carbonate (Ag2CO3) both form insoluble precipitates. However, silver nitrate does not precipitate with nitrate ions (NO3-) because all silver nitrates are soluble.

Examine the reaction with Pb(NO3)2

Lead(II) nitrate (Pb(NO3)2) is known to form precipitates with carbonate ions (CO3^2-) as lead carbonate (PbCO3) is insoluble. Lead bromide (PbBr2) also precipitates, though it's slightly soluble. Again, lead nitrate will not form a precipitate with nitrate ions (NO3-) because they remain soluble.

Analyze the reaction with BaCl2

Barium chloride (BaCl2) can form a precipitate with carbonate ions (CO3^2-) as barium carbonate (BaCO3) is insoluble in water. However, barium bromide and barium nitrate are soluble, so no precipitate forms with bromide (Br-) or nitrate (NO3-) ions.

Determine consistent anion

The only anion that forms a precipitate with all three reagents (AgNO3, Pb(NO3)2, BaCl2) is the carbonate ion (CO3^2-). Both Ag2CO3, PbCO3, and BaCO3 are insoluble, confirming the presence of CO3^2-.

Key concepts

Anion Identification

Identifying the anion in an unknown ionic compound is key when you're trying to predict how it will behave in chemical reactions. Anions are negatively charged ions that combine with positively charged ions to form ionic compounds. In our task, we have tested a solution with three different chemicals:

• Silver nitrate (\(\mathrm{AgNO}_{3}\))
• Lead(II) nitrate (\(\mathrm{Pb(NO}_{3})_{2}\))
• Barium chloride (\(\mathrm{BaCl}_{2}\))

Each chemical can help us identify the anion based on whether or not a precipitate forms when it is added to the solution. In this exercise, precipitates form with all three tests, singling out carbonate ions (\(\mathrm{CO}_{3}^{2-}\)) as the anion in our unknown ionic compound. Observing which anions react with specific cations can help us pinpoint the identity of the anion.

Precipitation Reactions

Precipitation occurs when ions in a solution react to form an insoluble solid, called a precipitate. It's a handy method for identifying ions in a solution. Precipitate formation happens based on solubility rules. When silver nitrate (\(\mathrm{AgNO}_{3}\)) reacts with bromide (\(\mathrm{Br}^{-}\)) or carbonate (\(\mathrm{CO}_{3}^{2-}\)) ions, it forms either silver bromide (\(\mathrm{AgBr}\)) or silver carbonate (\(\mathrm{Ag}_{2}\mathrm{CO}_{3}\)), both of which are insoluble. Similarly, lead(II) nitrate (\(\mathrm{Pb(NO}_{3})_{2}\)) reacts with bromide or carbonate ions to form lead bromide (\(\mathrm{PbBr}_{2}\)) and lead carbonate (\(\mathrm{PbCO}_{3}\)), which also precipitate out of solution. Finally, barium chloride (\(\mathrm{BaCl}_{2}\)) forms barium carbonate (\(\mathrm{BaCO}_{3}\)) when it reacts with carbonate ions, leading to precipitation. Here, none of these substances form a precipitate with nitrate ions (\(\mathrm{NO}_{3}^{-}\)), as they remain soluble.

Solubility Rules

Solubility rules are guidelines that help predict the solubility of different ionic compounds in water. These rules aid in determining whether a precipitate will form in a reaction. Knowing that most nitrate (\(\mathrm{NO}_{3}^{-}\)) salts are soluble while many carbonates (\(\mathrm{CO}_{3}^{2-}\)) are insoluble, can effectively guide predictions. For example, the general rule indicates that most silver, lead, and barium compounds with carbonate ions will produce an insoluble product.

• Silver compounds: Insoluble with bromides and carbonates, but not with nitrates.
• Lead compounds: Insoluble with bromides (except slightly) and carbonates, but soluble with nitrates.
• Barium compounds: Soluble with bromides and nitrates, but insoluble with carbonates.

Understanding and applying these solubility rules can help you anticipate which compounds will form in a chemical solution.