Like sulfuric acid, selenic acid H2SeO4 is a diprotic acid that has a very large value of Ka1. Calculate the pH and the concentrations of all species present in 0.50 M H2SeO4 (Ka2 = 1.2 * 10^-22).

Verified step by step guidance

1. Step 1: Recognize that H2SeO4 is a diprotic acid, meaning it can donate two protons (H+). The dissociation occurs in two steps: first, H2SeO4 dissociates into H+ and HSeO4-, and then HSeO4- dissociates into H+ and SeO4^2-.
2. Step 2: Since Ka1 is very large, assume the first dissociation is complete. Therefore, the concentration of H+ and HSeO4- after the first dissociation is approximately equal to the initial concentration of H2SeO4, which is 0.50 M.
3. Step 3: For the second dissociation, use the given Ka2 value (1.2 * 10^-22) to set up the equilibrium expression: Ka2 = [H+][SeO4^2-]/[HSeO4-]. Since the concentration of H+ from the first dissociation is much larger than the contribution from the second dissociation, assume [H+] ≈ 0.50 M.
4. Step 4: Substitute the known values into the equilibrium expression to solve for [SeO4^2-]. Assume [HSeO4-] ≈ 0.50 M due to the negligible change from the second dissociation. This simplifies the expression to Ka2 = [SeO4^2-] * 0.50 M.
5. Step 5: Calculate the pH using the concentration of H+ from the first dissociation. Since [H+] ≈ 0.50 M, use the formula pH = -log[H+] to find the pH.

Key Concepts

Diprotic Acids

Diprotic acids are acids that can donate two protons (H⁺ ions) per molecule in an aqueous solution. This characteristic allows them to undergo two dissociation steps, each with its own acid dissociation constant (Ka). Understanding the behavior of diprotic acids is crucial for calculating pH and species concentrations, as each step influences the overall acidity and the resulting equilibrium concentrations.

Acid Dissociation Constants (Ka)

The acid dissociation constant (Ka) quantifies the strength of an acid in solution, representing the equilibrium between the undissociated acid and its ions. A larger Ka value indicates a stronger acid that dissociates more completely. For diprotic acids, both dissociation steps have distinct Ka values, which are essential for determining the concentrations of all species in solution, especially when calculating pH.

pH Calculation

pH is a measure of the hydrogen ion concentration in a solution, calculated using the formula pH = -log[H⁺]. For strong acids, the pH can often be determined directly from the concentration of the acid, while for weak acids, it requires consideration of the dissociation constants. In the case of diprotic acids like selenic acid, both dissociation steps must be accounted for to accurately calculate the pH and the concentrations of all species present.