Problem 28 - Chemistry: The Central Science in SI Units, Global Edition

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: \(\mathrm{Br}^{-}, \mathrm{CO}_{3}^{2-}, \mathrm{NO}_{3}^{-} ?\)

Step-by-Step Solution

Answer

Based on the analysis, the anion present in the unknown salt must be either \(\mathrm{Br}^{-}\) or \(\mathrm{CO}_{3}^{2-}\).

Step 1: List the possible products it can form with AgNO3, Pb(NO3)2, and BaCl2

We will write down the possible products that can be formed when the unknown salt (which contains one of the anions given - \(\mathrm{Br}^{-}, \mathrm{CO}_{3}^{2-}, \mathrm{NO}_{3}^{-}\)) encounters each of the other ions present in the added solutions. For AgNO3: - \(\mathrm{AgBr}\) - \(\mathrm{Ag}_{2}\mathrm{CO}_{3}\) - \(\mathrm{AgNO}_{3}\) For Pb(NO3)2: - \(\mathrm{PbBr}_{2}\) - \(\mathrm{PbCO}_{3}\) - \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) For BaCl2: - \(\mathrm{BaBr}_{2}\) - \(\mathrm{BaCO}_{3}\) - \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\)

Step 2: Determine if precipitations form in each case using solubility rules

Now, we will use solubility rules to determine which of these possible products form a precipitate. Common solubility rules include: - All nitrates are soluble. - All compounds with group 1 metals are soluble. - All chlorides, bromides, and iodides are soluble, except Ag, Pb, and Hg compounds. - Most carbonates are insoluble, except group 1 metals and ammonium compounds. Using these solubility rules: For AgNO3: - \(\mathrm{AgBr}\) forms a precipitate - \(\mathrm{Ag}_{2}\mathrm{CO}_{3}\) forms a precipitate - \(\mathrm{AgNO}_{3}\) is soluble For Pb(NO3)2: - \(\mathrm{PbBr}_{2}\) forms a precipitate - \(\mathrm{PbCO}_{3}\) forms a precipitate - \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) is soluble For BaCl2: - \(\mathrm{BaBr}_{2}\) forms a precipitate - \(\mathrm{BaCO}_{3}\) forms a precipitate - \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\) is soluble

Step 3: Compare the formed precipitates with the given information

According to the given information, when the unknown salt is treated with each of the three solutions, a precipitate is formed in all cases. We can use this information to determine which anion is likely present in the unknown salt. Based on our results from Step 2, we see that when anions \(\mathrm{Br}^{-}\) and \(\mathrm{CO}_{3}^{2-}\) are present, precipitates form in all cases with each of the three solutions. However, when the \(\mathrm{NO}_{3}^{-}\) anion is present, it does not form a precipitate with any of the solutions. Therefore, it is not possible for the unknown salt to contain the \(\mathrm{NO}_{3}^{-}\) anion.

Step 4: Conclusion

Based on the analysis, the anion present in the unknown salt must be either \(\mathrm{Br}^{-}\) or \(\mathrm{CO}_{3}^{2-}\).

Key Concepts

Precipitation Reactions

Precipitation reactions occur when two soluble salts in aqueous solutions combine to form an insoluble salt, known as a precipitate. These reactions are quite common in chemical analysis because they allow scientists to identify certain ions in a solution by observing the formation of a solid.

For example, when you mix solutions containing silver nitrate (\(\text{AgNO}_3\)) and sodium chloride (\(\text{NaCl}\)), a white precipitate of silver chloride (\(\text{AgCl}\)) forms. The rule here is simple: when the ions of certain salts combine, they might create a solid product.

Understanding the specific conditions and combinations that lead to the formation of precipitates is crucial for identifying unknown substances.

Ionic Compounds

Ionic compounds are formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). They are typically formed when metals react with nonmetals, transferring electrons to achieve full outer electron shells.

Ionic compounds have unique properties, such as high melting and boiling points. They also tend to dissolve in water, where they can dissociate into their respective ions, a key factor in precipitation reactions.

When conducting experiments with ionic compounds, such as reacting an unknown ionic substance with \(\text{AgNO}_3\), \(\text{Pb(NO}_3)_2\), or \(\text{BaCl}_2\), understanding the ionic makeup can help predict potential reactions and identify precipitates like \(\text{AgBr}\), \(\text{PbCO}_3\), and \(\text{BaCO}_3\).

Chemical Analysis

Chemical analysis involves techniques and methods used to identify the composition of substances. By understanding the reactions that certain compounds undergo, including precipitation reactions, scientists can deduce which ions are present or absent in unknown samples.

For instance, if you want to identify the anion in an unknown ionic compound, you can react it with a series of known solutions. Observing which combinations produce precipitates will give clues about the identity of the anion.

In the analyzation process mentioned in the exercise, the reaction with \(\text{AgNO}_3\), \(\text{Pb(NO}_3)_2\), and \(\text{BaCl}_2\) helped narrow down the possibilities of the unknown anion, ruling out \(\text{NO}_3^-\) and pinpointing \(\text{Br}^-\) or \(\text{CO}_3^{2-}\) as the probable candidates. Such deductive techniques form the backbone of chemical analysis in both research and industrial applications.