Acid strength and buffering — Which statement correctly characterizes weak acids in terms of pKa and practical buffering use in biological systems?

Introduction / Context:
pKa quantifies acid strength: the lower the pKa, the stronger the acid. Biological buffers rely on weak acids and their conjugate bases to resist pH changes near physiological conditions (roughly pH 6–8).

Given Data / Assumptions:

• Weak acids are only partially dissociated in water.
• We seek a general statement about their pKa and utility.
• Biological systems abound with weak acids (amino acids, phosphate, organic acids).

Concept / Approach:
Weak acids typically have pKa values above ~2, often in the range 2–10. Effective buffering occurs within ~±1 pH unit of the pKa, so weak acids are ideal for physiological pH control. Very strong acids (pKa < 0–1) are fully dissociated in water and not useful as biological buffers.

Step-by-Step Solution:

Verification / Alternative check:
Tabulated pKa values of common biochemical acids (acetic acid 4.76, dihydrogen phosphate 7.2, lactic acid 3.86) support this.

Why Other Options Are Wrong:

• pKa < 1: describes strong acids, not weak.
• “Seldom found”: false; weak acids are ubiquitous in metabolism.
• “Cannot be used to buffer”: false; they are the primary buffering agents.

Common Pitfalls:
Assuming a single pKa describes polyprotic systems without considering multiple pKa values.

Final Answer:
pKa values greater than 2