A student titrates a \(20.0\text{ mL}\) sample of a weak monoprotic acid, \(\text{HA(aq)}\), with a standardized \(0.100\text{ M }\text{NaOH(aq)}\) solution at \(25^\circ\text{C}\). The titration curve indicates that the equivalence point is reached after exactly \(18.0\text{ mL}\) of \(\text{NaOH(aq)}\) is added. At the half-equivalence point, after \(9.0\text{ mL}\) of \(\text{NaOH(aq)}\) is added, the \(\text{pH}\) of the solution is \(4.00\). Based on these data, what is the concentration of \(\text{H}_3\text{O}^+\text{(aq)}\) in the original \(\text{HA(aq)}\) sample before any \(\text{NaOH(aq)}\) was added?