What is the pH of a 0.20 M H₂Se solution that has the stepwise dissociation constants K_a1 = 1.3 × 10^-4 and K_a2 = 1.0 × 10^-11? A)2.29 B)3.89 C)4.59 D)5.57

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Quizplus · Accepted Answer

Given the two stepwise dissociation constants (Ka1 = 1.3 × 10^-4 and Ka2 = 1.0 × 10^-11) for a dibasic acid, the first dissociation step is much stronger than the second. Therefore, the pH is primarily determined by the first dissociation. The pH can be approximated using the formula for a weak acid: pH = 1/2(pKa - log[C]), where C is the concentration of the acid. Substituting the given values: pH ≈ 1/2(log(1.3 × 10^-4) - log(0.20)) ≈ 2.29.

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