Consider the following salts. Which one(s) when dissolved in water will produce an acidic solution? 1\. \(\mathrm{NH}_{4} \mathrm{Cl}\) 2. \(\dot{\mathrm{K}} \mathrm{HSO}_{4}\) 3\. \(\mathrm{NaCN}\) 4. \(\mathrm{KNO}_{3}\) (a) 2 and 3 (b) 1 and 2 (c) only 3 (d) 2 and 4

Understanding the acid-base properties of salts is fundamental in predicting the pH of their aqueous solutions. Salts are formed by the neutralization reaction between an acid and a base, and their acidic or basic nature depends on the strength of the parent acid and base.

When dissolved in water, salts may exhibit neutral, acidic, or basic properties:

• Salts from a strong acid and a strong base (e.g., NaCl) do not affect the pH and form neutral solutions.
• Salts from a strong acid and a weak base (e.g., \(\mathrm{NH}_{4}\mathrm{Cl}\)) will release \(\mathrm{H}^{+}\) ions into the solution, making it acidic.
• Salts from a weak acid and a strong base (e.g., \(\mathrm{NaCN}\)) tend to increase the pH of the solution, creating basic conditions.
• If either the cation or the anion can react with water (hydrolysis), it will influence the solution's pH accordingly.

The pH of the resulting solution is thus a product of the ions' tendencies to donate or accept \(\mathrm{H}^{+}\) ions when hydrolyzed.

The pH of a salt solution can provide a quick insight into its acid-base character. Neutral salts yield a pH close to 7, while acidic salts lead to a pH less than 7, and basic salts will have a pH greater than 7.

The pH can be estimated or precisely calculated depending on the ion concentration and degree of ionization or hydrolysis. For example, \(\mathrm{NH}_{4}\mathrm{Cl}\) in water will produce \(\mathrm{NH}_{4}^{+}\) ions, which hydrolyze to form \(\mathrm{NH}_{3}\) and \(\mathrm{H}^{+}\), lowering the pH. In contrast, \(\mathrm{NaCN}\) forms \(\mathrm{CN}^{-}\), which can take a proton from water to form \(\mathrm{HCN}\) and \(\mathrm{OH}^{-}\), increasing the pH.

For accurate pH predictions, particularly for weakly acidic or basic ions, an equilibrium calculation involving the dissociation constants of the parent acid or base may be required.

Hydrolysis is the reaction of ions with water to form new substances that can affect the pH. It's crucial to understand that not all ions will undergo hydrolysis; it typically involves the ions from a weak acid or a weak base.

For instance, when \(\mathrm{NH}_{4}\mathrm{Cl}\) undergoes hydrolysis, the \(\mathrm{NH}_{4}^{+}\) ion reacts with water to release \(\mathrm{H}^{+}\) ions, which is why the solution becomes acidic. On the other hand, anions like \(\mathrm{CN}^{-}\) from \(\mathrm{NaCN}\) will attract protons from water molecules, forming \(\mathrm{HCN}\) and increasing the concentration of \(\mathrm{OH}^{-}\) in the solution, thus making it basic.

These hydrolysis reactions are governed by the ions' acid dissociation constant \(\mathrm{(K}_{a}\)) or base dissociation constant \(\mathrm{(K}_{b}\)), and they establish an equilibrium that is used to calculate the final pH of the solution.