A buffer that is most effective at a pH of about 4.5 is;
**Core Concept**
The question tests the understanding of buffer solutions and their effectiveness at different pH levels. A buffer is a solution that resists changes in pH when acids or bases are added, maintaining a relatively stable pH. The effectiveness of a buffer depends on its capacity to neutralize added hydrogen or hydroxide ions.
**Why the Correct Answer is Right**
A buffer that is most effective at a pH of about 4.5 is the acetate buffer. The acetate buffer consists of acetic acid (CH3COOH) and sodium acetate (CH3COONa). At a pH of 4.5, the acetate buffer is optimal because it has a high concentration of the conjugate base (acetate ions) that can neutralize added hydrogen ions. The Henderson-Hasselbalch equation (pH = pKa + log10([A-]/[HA])) explains this relationship, where pKa is the acid dissociation constant of acetic acid, and [A-] and [HA] are the concentrations of acetate ions and acetic acid, respectively.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.
**Clinical Pearl / High-Yield Fact**
The acetate buffer is commonly used in medical and biological applications, such as maintaining the pH of blood samples and preserving the stability of enzymes and proteins. It is essential to choose the correct buffer for a specific application based on the desired pH range.
**Correct Answer: C. Acetate buffer.**