Determine the pH of each solution. a. 0.20 M KCHO₂ b. 0.20 M CH₃NH₃I c. 0.20 M KI

Verified step by step guidance

1. Step 1: Identify the nature of each compound in the solution. KCHO_2 is a salt of a weak acid (HCHO_2, formic acid) and a strong base (KOH), CH_3NH_3I is a salt of a weak base (CH_3NH_2, methylamine) and a strong acid (HI), and KI is a salt of a strong acid (HI) and a strong base (KOH).
2. Step 2: For KCHO_2, determine the pH by considering the hydrolysis of the formate ion (CHO_2^-). Use the formula for the hydrolysis of anions: \( K_b = \frac{K_w}{K_a} \) where \( K_w \) is the ion-product constant of water and \( K_a \) is the acid dissociation constant of formic acid. Calculate \( [OH^-] \) and then find pOH and pH.
3. Step 3: For CH_3NH_3I, determine the pH by considering the hydrolysis of the methylammonium ion (CH_3NH_3^+). Use the formula for the hydrolysis of cations: \( K_a = \frac{K_w}{K_b} \) where \( K_b \) is the base dissociation constant of methylamine. Calculate \( [H^+] \) and then find pH.
4. Step 4: For KI, recognize that it is a neutral salt formed from a strong acid and a strong base, so it does not affect the pH of the solution. The pH of the solution is neutral, which is 7 at 25°C.
5. Step 5: Summarize the pH values: KCHO_2 solution will be basic, CH_3NH_3I solution will be acidic, and KI solution will be neutral.

Key Concepts

Acid-Base Chemistry

Acid-base chemistry involves the study of substances that can donate protons (acids) or accept protons (bases). The pH scale, which ranges from 0 to 14, quantifies the acidity or basicity of a solution, with lower values indicating higher acidity and higher values indicating higher basicity. Understanding the nature of the solutes in the question is essential for determining their contributions to the pH of the solutions.

Weak Acids and Bases

Weak acids and bases do not completely dissociate in solution, leading to an equilibrium between the undissociated species and the ions produced. For example, potassium formate (KCHO2) is the salt of a weak acid (formic acid), while methylammonium iodide (CH3NH3I) is the salt of a weak base (methylamine). The pH of solutions containing these compounds can be calculated using the dissociation constants (Ka or Kb) of the respective weak acids or bases.

Salt Hydrolysis

Salt hydrolysis occurs when the ions of a salt react with water to produce either acidic or basic solutions. In the case of KCHO2, the formate ion can hydrolyze to produce hydroxide ions, resulting in a basic solution. Conversely, the methylammonium ion from CH3NH3I can release protons, leading to an acidic solution. Understanding how these ions interact with water is crucial for calculating the pH of the solutions.