Question

Buffers are considered to be effective when the ratio of [HA] to [A-] is between

a. 0.5 and 1.5
b. 1 and 2
c. 2 and 3
d. 3 and 4

Answer 1

Buffers are most effective when the ratio of [HA] to [A-] is between 0.5 and 1.5, ensuring that the pH stays within one unit of the weak acid's pKa. This corresponds to an option (a), which ensures the maximum buffering capacity for the system. Option A is correct answer.

Step-by-step explanation:

Understanding Buffers and Their Effective Range

Buffers are solutions that maintain a relatively constant pH when an acid or a base is added, which is essential for various biological and chemical processes. A typical buffer system consists of a weak acid (HA) and its conjugate base (A-). Their ability to resist pH changes is rooted in the equilibrium established between these two components. When choosing an effective buffer, several factors need to be taken into account.

General Attributes of Buffers

Here are four key points about buffer systems:

1. A buffer system is composed of a weak acid and its conjugate weak base.
2. The pH control is greater with higher concentrations of buffer components.
3. The maximum pH control is achieved when the concentration of the weak acid equals that of its conjugate base ([HA] = [A-]), making the pH equal to the pKa of the weak acid.
4. For optimal buffering effect, the pKa of the weak acid should be close to the target pH, typically within one unit.

Effective Buffer Ratio

For a buffer to be effective, the ratio of the conjugate base to the weak acid, [A-]/[HA], should be within a specific range. This is because the pH of the buffer depends only on this ratio, not the absolute concentrations of the buffering components. According to the Henderson-Hasselbalch equation, when the [A-]/[HA] ratio is between 1 and 10, the pH will be within one pH unit above the pKa. Conversely, when this ratio is between 0.1 and 1, the pH will be within one pH unit below the pKa of the acid.

Therefore, to answer your question, buffers are considered to be effective when the ratio of [HA] to [A-] is between 1 and 2, which corresponds to an [A-]/[HA] ratio of between 0.5 and 1. This range maintains the pH within one unit of the pKa, ensuring maximum buffering capacity. Hence, option (a) 0.5 and 1.5 is the correct choice.