The pH of an acid solution is 2.15. If the concentration of the acid in solution is 0.100 mol/L, what is the Ka of the acid?
a) 1.8 x 10⁻⁵
b) 1.5 x 10⁻⁶
c) 2.0 x 10⁻⁴
d) 2.5 x 10⁻³