Calculate the [H₃O⁺] and pH of each polyprotic acid solution. b. 0.350 M H₂C₂O₄

Key Concepts

Polyprotic Acids

Polyprotic acids are acids that can donate more than one proton (H+) per molecule in a solution. Each dissociation occurs in a stepwise manner, with each step having its own dissociation constant (Ka). For example, oxalic acid (H2C2O4) can lose two protons, resulting in H2C2O4 → H+ + HC2O4- and then HC2O4- → H+ + C2O4^2-. Understanding these steps is crucial for calculating the concentrations of H3O+ and pH.

Dissociation Constants (Ka)

The dissociation constant (Ka) quantifies the strength of an acid in solution, indicating how readily it donates protons. For polyprotic acids, there are multiple Ka values corresponding to each dissociation step. The first dissociation typically has a larger Ka than subsequent ones, meaning it is more favorable. These constants are essential for calculating the concentrations of H3O+ ions and determining the pH of the solution.

pH Calculation

pH is a measure of the acidity of a solution, defined as the negative logarithm of the hydronium ion concentration: pH = -log[H3O+]. For polyprotic acids, the pH is influenced by the contributions from all dissociation steps. To calculate the pH accurately, one must consider the equilibrium concentrations of H3O+ from each dissociation and apply the appropriate Ka values to find the total concentration of H3O+ in the solution.