Which of the following expression(s) for the degree of dissociation of weak monobasic acid in aqueou — Ionic Equilibrium Chemistry Question

💡 Solution & Explanation

Degree of dissociation $\alpha = \frac{[\text{A}^-]}{C}$. Using $K_a = \frac{[\text{H}^+][\text{A}^-]}{[\text{HA}]}$ and $[\text{HA}] + [\text{A}^-] = C$, we substitute $[\text{HA}] = C - [\text{A}^-]$ to get $K_a(C - [\text{A}^-]) = [\text{H}^+][\text{A}^-]$. Rearranging gives $\alpha = \frac{[\text{A}^-]}{C} = \frac{K_a}{K_a + [\text{H}^+]}$ (Option B is correct). Option D simplifies to $\frac{1}{1 + [\text{H}^+]/K_a} = \frac{K_a}{K_a + [\text{H}^+]}$, which is also correct. Option A ($\sqrt{K_a/C}$) is the standard approximation for very weak acids. Option C represents the fraction of undissociated acid $[\text{HA}]/C$, making it the only definitively incorrect expression for degree of dissociation. Therefore, correct answer is C.