The weak ionization constant \( \left(\mathrm{K}_{\mathrm{a}}\right) \) for \( \mathrm{HC}_{2} \mathrm{O}_{4}{ }^{-} \)is equal to:
\[
\frac{\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\left[\mathrm{C}_{2} \mathrm{O}_{4}^{-}\right]}{\left[\mathrm{HC}_{2} \mathrm{O}_{4}{ }^{-}\right]} \quad \frac{\left[\mathrm{HC}_{2} \mathrm{O}_{4}^{-}\right]}{\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]\left[\mathrm{C}_{2} \mathrm{O}_{4}^{-}\right]}
\]