Determine whether aqueous solutions of the following salts...

Determine whether aqueous solutions of the following salts are acidic, basic, or neutral: $$ \begin{array}{l}{\text { (a) } \mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}} \\ {\text { (b) RbI }} \\ {\text { (c) } \mathrm{KHCO}_{2}} \\ {\text { (d) } \mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Br}}\end{array} $$

Determine whether aqueous solutions of the following salts are acidic, basic, or neutral:
$$
\begin{array}{l}{\text { (a) } \mathrm{Al}\left(\mathrm{NO}_{3}\right)_{3}} \\ {\text { (b) RbI }} \\ {\text { (c) } \mathrm{KHCO}_{2}} \\ {\text { (d) } \mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Br}}\end{array}
$$

Key Concepts

Amphiprotic Ions

Amphiprotic ions, such as the bicarbonate ion (HCO3–), are capable of acting as either an acid or a base when dissolved in water. Their behavior in solution depends on the relative strengths of the acid and base reactions. Typically, in salts where the cation does not affect pH, amphiprotic anions derived from weak acids yield basic solutions as the equilibrium favors the formation of OH–.

Conjugate Acid-Base Pairs

When a salt is formed from a weak acid or weak base, its ions may undergo hydrolysis. The anion of a weak acid can accept protons to form OH–, making the solution basic, while the cation of a weak base can donate protons to form H+, leading to an acidic solution. The nature of the original acid or base determines the hydrolytic behavior of the resulting salt.

Metal Cation Hydrolysis

Metal cations, especially those with high charge density like Al3+, tend to hydrolyze water due to their strong polarizing power. They can act as Lewis acids, binding water molecules and causing them to release protons. This results in the generation of H+ ions and an overall acidic solution.

Salt Hydrolysis

Salt hydrolysis is the process by which the cations and anions of a dissolved salt react with water to produce either H+ or OH– ions. The resulting change in hydrogen ion concentration determines whether the solution is acidic, basic, or neutral. This concept is fundamental to understanding the behavior of salts in aqueous solutions.

Spectator Ions and Neutrality

Ions that originate from strong acids (like NO3– from HNO3) or strong bases (such as Rb+ from RbOH) do not undergo significant hydrolysis. They do not react with water to affect the pH substantially, and as a result, salts composed solely of these ions yield neutral solutions.

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