Calculate the pH of a solution that contains 3.9 × 10^-4 M H₃O^⁺ at 25°C. A)4.59 B)3.41 C)10.59 D)9.41 E)0.59

Question

Quizplus · Accepted Answer

The dissociation reaction of HS⁻ is:

HS⁻ (aq) + H₂O (l) ⇌ H₂S (aq) + OH⁻ (aq)

The equilibrium constant expression is:

Kw/Ka = ([H₂S][OH⁻])/[HS⁻]

At 25°C, Kw = 1.0 × 10⁻¹⁴ and Ka = 1.2 × 10⁻⁷ for HS⁻.

Substituting the given concentrations into the expression above and solving for [H⁺], we get:

[H⁺] = Ka × [HS⁻]/[H₂S] = 1.2 × 10⁻⁷ × 3.9 × 10⁻⁴/1 = 4.68 × 10⁻¹¹ M

Taking the negative logarithm of this value gives:

pH = -log[H⁺] = -log(4.68 × 10⁻¹¹) = 3.41

Therefore, the answer is B.

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