14.5 Polyprotic Acids

Which of the following concentrations would be practically equal in a calculation of the equilibrium concentrations in a 0.134-M solution of H₂CO₃, a diprotic acid: $\left[\text{H}_{3} \text{O}^{+}\right] ,$ [OH⁻], [H₂CO₃], $\left[\text{HCO}_{3} ^{−}\right] ,$ $\left[\text{CO}_{3} ^{2−}\right] ?$ No calculations are needed to answer this question.

Calculate the concentration of each species present in a 0.010-M solution of phthalic acid, C₆H₄(CO₂H)₂.
$\begin{matrix} \\ \text{C}_{6} \text{H}_{4} \left(\text{CO}_{2} \text{H}\right)_{2} \left(a q\right) + \text{H}_{2} \text{O} \left(l\right) \rightleftharpoons \text{H}_{3} \text{O}^{+} \left(a q\right) + \text{C}_{6} \text{H}_{4} \left(\text{CO}_{2} \text{H}\right) \left(\text{CO}_{2}\right)^{−} \left(a q\right) K_{\text{a}} = 1.1 \times 10^{−3} \\ \text{C}_{6} \text{H}_{4} \left(\text{CO}_{2} \text{H}\right) \left(\text{CO}_{2}\right) ^{–} \left(a q\right) + \text{H}_{2} \text{O} \left(l\right) \rightleftharpoons \text{H}_{3} \text{O}^{+} \left(a q\right) + \text{C}_{6} \text{H}_{4} \left(\text{CO}_{2}\right)_{2} ^{2−} \left(a q\right) K_{\text{a}} = 3.9 \times 10^{−6}\end{matrix}$

The ion HTe⁻ is an amphiprotic species; it can act as either an acid or a base.

(a) What is K_a for the acid reaction of HTe⁻ with H₂O?

(b) What is K_b for the reaction in which HTe⁻ functions as a base in water?

(c) Demonstrate whether or not the second ionization of H₂Te can be neglected in the calculation of [HTe⁻] in a 0.10 M solution of H₂Te.