Consider a solution formed by mixing \(100.0 \mathrm{mL}\) of \(0.10 \mathrm{M}\) HA \(\left(K_{\mathrm{a}}=1.0 \times 10^{-6}\right), 100.00 \mathrm{mL}\) of \(0.10 M \mathrm{NaA},\) and \(100.0 \mathrm{mL}\) of \(0.10 \mathrm{M}\) HCl. In calculating the \(\mathrm{pH}\) for the final solution, you would make some assumptions about the order in which various reactions occur to simplify the calculations. State these assumptions. Does it matter whether the reactions actually occur in the assumed order? Explain.

In this problem, we make two assumptions when calculating the final pH of the solution. First, we assume that the strong acid HCl reacts with the conjugate base A- before the weak acid HA ionizes with water. Second, we assume that after the reaction between HCl and A-, the weak acid HA undergoes ionization with water to establish an equilibrium state, which can be determined by the Ka expression for HA. The order of reactions does not matter for calculating the final pH because the critical factor is to account for the neutralizing effect of the strong acid on the conjugate base first. Even if the weak acid ionization reaction occurs before the strong acid reaction, the final pH would still be the same since the concentrations of all components at equilibrium would be the same, regardless of the reaction order.