What is the pH of a 0.10 M H₂Se solution that has the stepwise dissociation constants K_a1 = 1.3 × 10^-4 and K_a2 = 1.0 × 10^-11? A)2.44 B)3.89 C)4.89 D)5.50

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Quizplus · Accepted Answer

The pH of the solution can be approximated using only the first dissociation constant (Ka1 = 1.3 × 10^-4) because the second dissociation constant (Ka2 = 1.0 × 10^-11) is much smaller and contributes negligibly to the overall H+ concentration. For a weak acid HA dissociating as HA ⇌ H+ + A^-, the pH can be estimated using the formula pH = 1/2(pKa - log[HA]), where [HA] is the concentration of the acid. Substituting the given values, pKa1 = -log(1.3 × 10^-4) ≈ 3.89, and [HA] = 0.10 M, we get pH ≈ 1/2(3.89 - log(0.10)) = 1/2(3.89 + 1) = 2.44.

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