Consider a 0.10 M solution of a weak polyprotic acid (H2A) with the possible values of Ka1 and Ka2 given here: b. Ka1 = 1.0 * 10^-4; Ka2 = 1.0 * 10^-5. Calculate the contributions to [H3O+] from each ionization step. At what point can the contribution of the second step be neglected? c. Ka1 = 1.0 * 10^-4; Ka2 = 1.0 * 10^-6.

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Identify the ionization steps for the polyprotic acid H2A. The first ionization step is H2A ⇌ H+ + HA-, and the second ionization step is HA- ⇌ H+ + A2-.

For the first ionization step, use the expression for the acid dissociation constant: Ka1 = [H+][HA-]/[H2A]. Assume initial concentrations and solve for [H+].

For the second ionization step, use the expression for the acid dissociation constant: Ka2 = [H+][A2-]/[HA-]. Use the [H+] from the first step to solve for the additional [H+] contributed by the second ionization.

Compare the [H+] contributions from the first and second ionization steps. Determine when the contribution from the second step is negligible by comparing the magnitude of Ka1 and Ka2.

For part c, repeat the process with the new value of Ka2 = 1.0 * 10^-6, and analyze the contributions to [H3O+] from each ionization step.>

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Polyprotic Acids

Polyprotic acids are acids that can donate more than one proton (H+) per molecule. Each ionization step has its own dissociation constant (Ka), which indicates the strength of the acid at that step. For example, H2A can lose its first proton to form HA-, and then lose a second proton to form A2-. Understanding the sequential ionization is crucial for calculating the contributions to hydronium ion concentration.

Dissociation Constants (Ka)

The dissociation constant (Ka) quantifies the extent to which an acid dissociates in solution. A higher Ka value indicates a stronger acid that dissociates more completely. In the case of polyprotic acids, the first dissociation (Ka1) is typically stronger than the second (Ka2), which affects the calculation of hydronium ion concentration from each ionization step. This concept is essential for determining the contributions of each step to the overall acidity of the solution.

Neglecting Contributions

In weak acid equilibria, contributions from subsequent ionization steps can often be neglected if they are significantly smaller than the contributions from the first step. This is typically assessed by comparing the Ka values and the concentrations involved. For example, if the concentration of H3O+ from the second ionization is less than 5% of that from the first, it can be considered negligible, simplifying calculations and analysis.

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Consider a 0.10 M solution of a weak polyprotic acid (H₂A) with the possible values of K_a1 and K_a2 given here.

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