Question

A weak acid HA has degree of dissociation x. Which option gives the correct expression of \(pH - pK_a\)?

• A. log(1 + 2x)
• B. 0
• C. log\(\left(\frac{x}{1-x}\right)\)
• D. log\(\left(\frac{1-x}{x}\right)\)

Hint:

For weak acids, dissociation can be quantified by the degree of dissociation x, which influences the pH relative to its \(pK_a\).

Answer 1

The Correct Option is A

The Henderson-Hasselbalch equation for a weak acid is \( pH = pK_a + \log \left( \frac{[A^-]}{[HA]} \right) \). When the dissociation degree is x, the equation simplifies to \( pH = pK_a + \log(1 + 2x) \). This assumes the weak acid dissociates into \(x\) molar concentration of H$^+$ and A$^-$ , with \(1 - x\) molar concentration of undissociated HA remaining.