Polyprotic acids Knowledge-Based Download PDF

Questions

1. 1.

   For an aqueous solution containing the divalent anion $\text{A}^{2-}$ derived from a weak diprotic acid $\text{H}_2\text{A}$, which expression correctly defines the first base dissociation constant, $K_{b1}$, in terms of the ion-product constant of water, $K_w$, and the stepwise acid dissociation constants of $\text{H}_2\text{A}$?

   • A) $K_{b1} = \frac{K_w}{K_{a1}}$
   • B) $K_{b1} = \frac{K_w}{K_{a2}}$
   • C) $K_{b1} = \frac{K_w}{K_{a1} K_{a2}}$
   • D) $K_{b1} = \frac{K_{a2}}{K_w}$
2. 2.

   Which expression gives the approximate hydronium ion concentration, $[H_3O^+]$, in an aqueous solution of the intermediate amphiprotic salt $NaHA$ derived from a weak diprotic acid, $H_2A$, assuming the analytical salt concentration $C$ is sufficiently high and water autoionization is negligible?

   • A) $[H_3O^+] \approx \sqrt{K_{a1} C}$
   • B) $[H_3O^+] \approx \frac{K_{a1} + K_{a2}}{2}$
   • C) $[H_3O^+] \approx \sqrt{K_{a1} K_{a2}}$
   • D) $[H_3O^+] \approx \sqrt{K_{a2} C}$
3. 3.

   Why is the second acid dissociation constant ($K_{a2}$) of a weak polyprotic acid typically several orders of magnitude smaller than its first acid dissociation constant ($K_{a1}$)?

   • A) The second ionization step generates hydroxide ions that inhibit further proton loss through the common-ion effect.
   • B) Electrostatic attraction makes removing a positively charged proton from a negatively charged anion significantly more difficult than from a neutral molecule.
   • C) The conjugate base produced in the first ionization step is insoluble in water, preventing further dissociation.
   • D) The activation energy for the first proton transfer is substantially higher than the activation energy of the second step.
4. 4.

   For a 0.10 M aqueous solution of a weak triprotic acid, $\text{H}_3\text{A}$, with successive dissociation constants $K_{a1} = 1.0 \times 10^{-3}$, $K_{a2} = 1.0 \times 10^{-7}$, and $K_{a3} = 1.0 \times 10^{-12}$, which of the following lists the acid-derived species in order of decreasing equilibrium concentration?

   • A) $[\text{A}^{3-}] > [\text{HA}^{2-}] > [\text{H}_2\text{A}^-] > [\text{H}_3\text{A}]$
   • B) $[\text{H}_2\text{A}^-] > [\text{HA}^{2-}] > [\text{A}^{3-}] > [\text{H}_3\text{A}]$
   • C) $[\text{H}_3\text{A}] > [\text{H}_2\text{A}^-] > [\text{HA}^{2-}] > [\text{A}^{3-}]$
   • D) $[\text{H}_3\text{A}] > [\text{HA}^{2-}] > [\text{H}_2\text{A}^-] > [\text{A}^{3-}]$
5. 5.

   For an aqueous solution of a weak diprotic acid, $H_2A$, with successive ionization constants $K_{a1}$ and $K_{a2}$ such that $K_{a1} \gg K_{a2}$, which expression correctly approximates the equilibrium concentration of the divalent anion, $[A^{2-}]$?

   • A) $[A^{2-}] \approx \sqrt{K_{a1} \cdot K_{a2}}$
   • B) $[A^{2-}] \approx \sqrt{K_{a2} \cdot [H_2A]_0}$
   • C) $[A^{2-}] \approx \frac{K_{a1}}{K_{a2}}$
   • D) $[A^{2-}] \approx K_{a2}$