Which of the following conditions and approximations correctly apply to the calculation of pH for a — Ionic Equilibrium Chemistry Question

💡 Solution & Explanation

For polyprotic acids, successive dissociation constants decrease drastically ($K_{a1} \gg K_{a2}$). Because of the common ion effect from the first dissociation, the second dissociation is heavily suppressed, contributing negligibly to $[H^+]$. Therefore, $[H^+]$ is determined solely by $K_{a1}$. The second dissociation $HA^- \rightleftharpoons H^+ + A^{2-}$ has $K_{a2} = \frac{[H^+][A^{2-}]}{[HA^-]}$. Since $[H^+] \approx [HA^-]$, they cancel out, leaving $[A^{2-}] \approx K_{a2}$.