Answer
$pH = 4.04$
Work Step by Step
1. Calculate the pKa Value
$pKa = -log(Ka)$
$pKa = -log( 1.8 \times 10^{- 4})$
$pKa = 3.74$
2. Using the Henderson–Hasselbalch equation:
$pH = pKa + log(\frac{[Base]}{[Acid]})$
$pH = 3.74 + log(\frac{0.5}{0.25})$
$pH = 3.74 + log(2)$
$pH = 3.74 + 0.301$
$pH = 4.04$
