Ease Into Prep with a DAT^® Question of the Day

The correct answer is A.

Calculations involving K_a, concentration, and percent ionization can be accomplished via the approximation K_a = x²/c₀, where x = [H⁺] and c₀ = [HA] for dilute solutions of the weak acid HA. Since the acid is 0.002% (or 0.00002) ionized in a 0.20 M solution, we can find x = [H⁺] as:

x = (0.00002)(0.20) = 4 x 10^-6 M

We can safely assume that c₀ - 0.002% c₀ = c₀ = 0.20 M. Substituting into the K_a expression yields: K_a = x²/c₀ = (4 x 10^-6)²/(0.20) = (16 x 10^-12)/(2 x 10-¹) = 8 x 10^-11.