Separate samples of a solution of an unknown salt are treated with dilute solutions of HBr, H2SO4, and NaOH. A precipitate forms in all three cases. Which of the following cations could be present in the unknown salt solution: K+, Pb2+, Ba2+?

Name: Chemistry: The Central Science
Author: Brown
ISBN: 9780137542970

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Identify the possible reactions that could occur when the unknown salt solution is treated with HBr, H2SO4, and NaOH.

Consider the solubility rules for common ionic compounds to determine which cations form precipitates with the given anions.

For HBr, check which cations form insoluble bromides. Pb^{2+} forms PbBr2, which is insoluble.

For H2SO4, check which cations form insoluble sulfates. Ba^{2+} forms BaSO4, which is insoluble.

For NaOH, check which cations form insoluble hydroxides. Pb^{2+} forms Pb(OH)2, which is insoluble.

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Precipitation Reactions

Precipitation reactions occur when two soluble salts react in solution to form an insoluble compound, known as a precipitate. The formation of a precipitate indicates that the ions in solution have combined to form a compound that is not soluble in water. Understanding the solubility rules helps predict whether a precipitate will form when different reagents are added to a solution.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H+) between reactants. In this context, HBr and H2SO4 are strong acids that can react with bases or certain metal cations to form soluble or insoluble salts. The ability of a cation to form a precipitate with an acid indicates its reactivity and solubility characteristics, which are essential for identifying the unknown salt.

Solubility Rules

Solubility rules are guidelines that predict the solubility of various ionic compounds in water. For example, most sulfates are soluble, except for those of barium, lead, and calcium, while hydroxides are generally insoluble except for alkali metals and some alkaline earth metals. These rules are crucial for determining which cations (K+, Pb2+, Ba2+) could be present in the unknown salt based on the observed precipitate formation.