Consider phosphorous acid, a polyprotic acid with the formula H3PO3. (a) Draw two plausible structures for H3PO3. For each one, predict the shape of the pH titration curve for the titration of H3PO3 (Ka1 = 1.0 * 10^-2) with aqueous NaOH. (b) For the structure with the H atoms in two different environments, calculate the pH at the first and second equivalence points, assuming that 30.00 mL of 0.1240 M H3PO3 (Ka2 = 2.6 * 10^-7) is titrated with 0.1000 M NaOH.

Polyprotic acids are acids that can donate more than one proton (H+) per molecule in a stepwise manner. Each dissociation step has its own acid dissociation constant (Ka), which indicates the strength of the acid at that stage. For phosphorous acid (H3PO3), the first dissociation is stronger than the second, leading to different pH behaviors during titration. Understanding the dissociation process is crucial for predicting the pH at various points in a titration curve.

A titration curve is a graphical representation of the pH of a solution as a function of the volume of titrant added. For polyprotic acids, the curve typically shows multiple inflection points corresponding to the different dissociation steps. The shape of the curve is influenced by the strength of the acid and the concentration of the titrant. Analyzing the curve helps in identifying equivalence points, where the amount of titrant equals the amount of acid present.

Equivalence points in a titration occur when the amount of titrant added is stoichiometrically equivalent to the amount of acid present in the solution. For polyprotic acids like H3PO3, there are multiple equivalence points corresponding to each proton donation. The pH at these points can be calculated using the concentrations of the acid and the base, as well as the dissociation constants. Understanding how to calculate pH at these points is essential for accurately interpreting titration results.