Answer
The pH of this lactic acid buffer solution is equal to $3.64$.
Work Step by Step
1. Calculate the pKa Value
$pKa = -log(Ka)$
$pKa = -log( 1.4 \times 10^{- 4})$
$pKa = 3.85$
2. Using the Henderson–Hasselbalch equation:
$pH = pKa + log(\frac{[Base]}{[Acid]})$
$pH = 3.85 + log(\frac{0.36}{0.57})$
$pH = 3.85 + log(0.6207)$
$pH = 3.85 + (-0.2071)$
$pH = 3.64$
