If the K_a of a weak acid is 4.0 x 10^-6, what is the K_b of its conjugate base at 25°C?

• A. K_b = 2.5 x 10^-7
• B. K_b = 2.5 x 10^-8
• C. K_b = 2.5 x 10^-6
• D. K_b = 2.5 x 10^-9

Answer: (D)