The pH of an acid solution is 2.15. If the concentration of the acid in the solution is 0.100 mol/L, what is the Ka of the acid?

a) 1.0 x 10^(-2.15)
b) 1.0 x 10^(-11)
c) 1.0 x 10^(-13.85)
d) 1.0 x 10^(-15)