Butanoic Acid Contributes to the Rancid Odor of Spoiled Butter $\times$

Butanoic acid contributes to the rancid odor of spoiled butter. Calculate the acid ionization constant for butanoic acid if a 0.155 M solution is 1.15% ionized. The abbreviated structural formula for butanoic acid is CH₃CH₂CH₂COOH.

A) 5.1 $\times$ 10^-3
B) 1.8 $\times$ 10^-3
C) 1.2 $\times$ 10^-2
D) 2.1 $\times$ 10^-5
E) 1.5 $\times$ 10^-5