Problem 11 - Chemistry: An Atoms-Focused Approach

Question

Why does a solution of a weak acid and its conjugate base control pH better than a solution of the weak acid alone?

Step-by-Step Solution

Answer

Answer: A solution of a weak acid and its conjugate base controls pH better than a solution of the weak acid alone because it has a buffering capacity. The conjugate base can neutralize any added strong acid, and the weak acid can neutralize any added strong base, maintaining a relatively constant pH. In contrast, a solution containing only a weak acid does not have the same buffering capacity and results in larger changes in pH when strong acidic or basic species are added.

Step 1: Understanding the concept of a buffer solution

A buffer solution is a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. It is designed to maintain a relatively constant pH even when small amounts of strong acidic or basic substances are added. This pH stability is due to the equilibrium between the weak acid (or base) and its conjugate species in the solution.

Step 2: Define the pH relationship in a buffer solution

The relationship between pH, the concentration of the weak acid [HA], and the concentration of its conjugate base [A-] in a buffer solution can be described using the Henderson-Hasselbalch equation: pH = pKa + log(\frac{[A-]}{[HA]}) where pKa is the negative logarithm of the acid dissociation constant (Ka) of the weak acid.

Step 3: Observing the pH change upon addition of a strong acid or base in a weak acid solution

If we have a solution containing only a weak acid and we add a small amount of a strong acid (e.g., HCl), the concentration of protons (H+) in the solution will increase. This change will result in a decrease in pH. Similarly, if we add a small amount of a strong base (e.g., NaOH), the concentration of hydroxide ions (OH-) will increase, neutralizing some of the H+ ions present in the solution, and thus increasing the pH.

Step 4: Examining the pH change upon addition of a strong acid or base in a buffer solution

In a buffer solution containing both the weak acid and its conjugate base, when a small amount of strong acid is added, it will react with the conjugate base (A-) to form the weak acid (HA) and water. Since the conjugate base absorbs and neutralizes the added H+ ions, the pH remains relatively constant. Similarly, when a small amount of strong base is added, it will react with the weak acid (HA) to form the conjugate base (A-) and water. This process neutralizes the added OH- ions and maintains the pH at a relatively constant level.

Step 5: Conclusion

A solution of a weak acid and its conjugate base controls pH better than a solution of the weak acid alone because the conjugate base can react with any added strong acid, and the weak acid can react with any added strong base. These reactions serve to neutralize the added acidic or basic species in the solution, maintaining a relatively constant pH. In contrast, a solution containing only a weak acid does not have the same buffering capacity, as it cannot effectively neutralize added strong acidic or basic species, resulting in larger changes in pH.