The pH of a \( 0.1\text{ M} \) monobasic acid solution is 4.50. What is the value of \( \text{p}K_a \) for this acid?

\( [\text{H}^+] = 10^{-4.5} = 3.16 \times 10^{-5}\text{ M} \). \( K_a = [\text{H}^+]^2/c = (3.16 \times 10^{-5})^2 / 0.1 = 1.0 \times 10^{-8} \). Thus, \( \text{p}K_a = -\log(10^{-8}) = 8 \).