Question

Please calculate the pH and [S2-] in a 0.15 M H₂S solution. Assume Ka1 = 1.0 x 10⁻⁷; Ka2 = 1.0 x 10⁻¹⁹. What is the pH? What is the concentration of S2-?

Answer 1

To calculate the pH of a 0.15 M H2S solution, consider only the first dissociation due to the small size of Ka2 compared to Ka1. Use Ka1 to find [H+], which gives the pH. The concentration of S2- is found using the product of Ka1 and Ka2.

Step-by-step explanation:

When calculating the pH and the concentration of S2- in a 0.15 M H2S solution, we need to consider the dissociation of H2S into its ions and the respective equilibrium constants Ka1 and Ka2. Since the second dissociation constant Ka2 is much smaller than Ka1 (1.0 x 10-19 versus 1.0 x 10-7), it is reasonable to assume that the contribution of the second ionization to the total [H+] is negligible. Therefore, we will only consider the first dissociation.

For the first dissociation of H2S, we have H2S ↔ H+ + HS-, with Ka1 = 1.0 x 10-7. Applying the equilibrium expression for this dissociation:

Ka1 = [H+][HS-]/[H2S]

Since [H2S] remains relatively constant due to the large excess, we can calculate [H+] and, subsequently, the pH. To find [S2-], we use the overall equilibrium constant K = Ka1 x Ka2, and the given concentration of H2S to find the concentration of S2-.