Chapter 16.5, Problem 2CP

A 0.042-M solution of a weak acid has pH 4.01 at 25°C. What is the $K_{\text{a}}$ of this acid?

$\begin{array}{l}\text{a})\\ 9.5\times {10}^{-9}\\ \text{b})\\ 9.8\times {10}^{-5}\\ \text{c})\\ 0.91\\ \text{d})\\ 4.2\times {10}^{-9}\\ \text{e})\\ 2.3\times {10}^{-7}\end{array}$