The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is 2.38. Calculate Ka for formic acid at this temperature.
a) 1.2 x 10⁻⁴
b) 2.5 x 10⁻⁴
c) 3.8 x 10⁻⁴
d) 5.1 x 10⁻⁴