Acid-Base Properties of Ions and Salts Determine the \(\mathrm{pH}\) of each solution. \(\begin{array}{l}{\text { a. } 0.20 \mathrm{M} \mathrm{KCHO}_{2}} \\ {\text { b. } 0.20 \mathrm{M} \mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{I}} \\ {\text { c. } 0.20 \mathrm{M} \mathrm{KI}}\end{array}\)

Understanding the acid-base properties of salts is essential for predicting the pH of a solution. When salts dissolve in water, their ions can react with water in a process called hydrolysis, which affects the pH of the solution.

Salts formed from a strong acid and a strong base, like potassium iodide (KI), do not undergo hydrolysis, so the solution remains neutral with a pH of 7. But for salts derived from a weak acid and a strong base, such as potassium formate (KCHO2), or from a strong acid and a weak base, like methylammonium iodide (CH3NH3I), hydrolysis occurs and the solution becomes either basic or acidic. The type of parent acid and base determines the resulting pH.

Hydrolysis of salts is the reaction of salt ions with water to produce acid or alkaline solutions. This process significantly influences the pH of salt solutions.

For example, the anion CHO2^- from KCHO2 hydrolyzes to form OH^- (increasing the pH), while the cation CH3NH3⁺ from CH3NH3I hydrolyzes to form H⁺ (decreasing the pH). In contrast, the ions from salts of strong acids and bases, like KI, do not hydrolyze, and the pH remains neutral. Understanding which ions will hydrolyze and how strongly determines the overall pH.

Calculating the pH of a solution is a fundamental skill in chemistry that involves determining the concentration of hydrogen ions (H⁺) or hydroxide ions (OH^-).

For the salt KCHO2, the hydrolysis of the formate ion (CHO2^-) produces OH^-, where the equilibrium concentration can be found using the Kb value from the dissociation of parent weak acid. Conversely, for CH3NH3I, the hydrolysis of CH3NH3⁺ produces H⁺, where Ka is used. The pH is then determined by converting these ion concentrations using the formulas pH = -log[H⁺] and pOH = -log[OH^-], and remembering that pH + pOH = 14 in aqueous solutions.

The strength of acids and bases is determined by their ability to dissociate in water. Strong acids and bases fully dissociate, while weak acids and bases only partially dissociate.

A strong acid like hydroiodic acid (HI) or a strong base like potassium hydroxide (KOH) will produce solutions with a neutral pH when they form salts, as they do not contribute to hydrolysis. On the other hand, salts derived from weak acids, such as formic acid (HCHO2), or weak bases, like methylamine (CH3NH2), will undergo hydrolysis because their ions are not completely neutralized and still possess reactive properties. Recognizing the strength of the parent acid or base from which a salt is derived is crucial for predicting the behavior of salts in aqueous solutions.