Calculate the pH of a solution that contains 3.9 10^-⁴M H⁺. A) 4.59 B) 10.59 C) 3.41 D) 9.41

Question

Quizplus · Accepted Answer

The given chemical equation is that of the reaction of hydrogen sulfide (HS-) with water (H2O) to form hydronium ions (H3O+) and sulfide ions (S2-).
HS- + H2O ⇌ H3O+ + S2-
The equilibrium constant expression for this reaction can be written as:
Kw/Ka2 = [H3O+][S2-]/[HS-][H2O]
At equilibrium, the amount of HS- will be equal to the amount of S2-, which can be expressed as [HS-] = [S2-] = x M.
Therefore, the equilibrium constant expression becomes:
Kw/Ka2 = (x M)(x M)/[(3.9 * 10^-11) M] = x^2/3.9 * 10^-11
Taking the square root of both sides and solving for x gives the concentration of hydronium ions to be 1.18 * 10^-7 M.
pH = -log[H3O+] = -log(1.18 * 10^-7) = 6.43
Since the pH is less than 7, the solution is acidic. The best choice is C.

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