acid base help?

Question

Butyric acid is responsible for the odor in rancid butter. A solution of 0.25 M butyric acid has a pH of 2.71. What is the Ka for the acid?

Answer 1

you will know at equilibrium
Ka=x*x/.25
ph=2.71
x=10^-2.71=.0019
Ka=.0019^2/.25= 1.59
good luck

Answer 2

ph=2.71
concentration of H+ ions in the solution is 10^-2.71
thus, Ka=(concentration of H+ ions )^2/concentration of butyric acid

thus Ka=10^-5.42/.25
Ka=4*10^-5.42