HZ Is a Weak Acid

HZ is a weak acid. An aqueous solution of HZ is prepared by dissolving 0.020 mol of HZ in sufficient water to yield 1.0 L of solution. The pH of the solution was 4.93 at 25.0 °C. The K_a of HZ is .

A) 1.4 × 10^{- 10}
B) 2.8 × 10^{- 12}
C) 1.2 × 10^{- 5}
D) 9.9 × 10^{- 2}
E) 6.9 × 10^{- 9}