You have to prepare a pH = 3.50 buffer, and you have the following 0.10 M solutions available: HCOOH, CH₃COOH, H₃PO₄, HCOONa, CH₃COONa, and NaH₂PO₄. Which solutions would you use?

Verified step by step guidance

1. Step 1: Understand the concept of a buffer solution. A buffer solution resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base.
2. Step 2: Identify the weak acids and their conjugate bases from the given solutions. The weak acids are HCOOH (formic acid), CH3COOH (acetic acid), and H3PO4 (phosphoric acid). Their conjugate bases are HCOONa (sodium formate), CH3COONa (sodium acetate), and NaH2PO4 (sodium dihydrogen phosphate), respectively.
3. Step 3: Use the Henderson-Hasselbalch equation to determine which buffer system can achieve the desired pH. The equation is: pH = pKa + log([A^-]/[HA]), where [A^-] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.
4. Step 4: Compare the pKa values of the weak acids to the desired pH of 3.50. The pKa of formic acid (HCOOH) is approximately 3.75, acetic acid (CH3COOH) is approximately 4.76, and the first dissociation of phosphoric acid (H3PO4) is approximately 2.15.
5. Step 5: Choose the buffer system where the pKa is closest to the desired pH of 3.50. This will provide the most effective buffering capacity around the target pH.

Key Concepts

Buffer Solutions

Buffer solutions are mixtures that resist changes in pH when small amounts of acid or base are added. They typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid. Understanding how buffers work is essential for preparing solutions with a specific pH, as they maintain the desired acidity or alkalinity.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the concentration of its acid and conjugate base. It is expressed as pH = pKa + log([A-]/[HA]), where pKa is the acid dissociation constant. This equation is crucial for determining the appropriate ratio of acid to base needed to achieve a specific pH, such as 3.50 in this case.

Weak Acids and Their Conjugate Bases

Weak acids partially dissociate in solution, establishing an equilibrium between the undissociated acid and its ions. The choice of weak acid and its conjugate base is vital for buffer preparation. In this scenario, HCOOH (formic acid) and HCOONa (sodium formate) or CH3COOH (acetic acid) and CH3COONa (sodium acetate) can be used to create a buffer at the desired pH.