A 2.5 M solution of a weak acid is 0.52% ionized. What is K_a for this acid? A) 1.3 × 10^-2 B) 6.8 × 10^-5 C) 0.11 D) 1.1 × 10^-5 E) none of these

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The ionization percentage indicates that only a small fraction of the acid dissociates into its ions. To find the acid dissociation constant (Ka), we use the formula Ka = [H+][A-]/[HA], where [H+] and [A-] are the concentrations of the hydrogen ions and the acid's conjugate base, respectively, and [HA] is the concentration of the undissociated acid. Given a 2.5 M solution with 0.52% ionization, the concentration of ionized acid ([H+] and [A-]) is 0.52% of 2.5 M, which is 0.013 M. The concentration of undissociated acid is approximately the initial concentration (2.5 M) since the ionization is very small. Therefore, Ka = (0.013)^2 / (2.5) = 6.8 × 10^-5, matching choice B.

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