A chemist is investigating the behavior of an equimolar mixture of hydrogen sulfide, H S
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A chemist is investigating the behavior of an equimolar mixture of hydrogen sulfide, H2S, and ethylamine, C2H5NH2, in water. Exactly 0.0500 mol of each substance is dissolved in enough water to make 1.00 L of solution at 25 °C.
Relevant data at 25 °C:
• Ka1(H2S) = 9.1×10−8
• Ka2(H2S) = 1.3×10−13
• Kb(C2H5NH2) = 6.4×10−4
• Kw = 1.0×10−14
(a) List all acid–base reactions that could plausibly occur in this solution, identifying the conjugate acid–base pairs involved.
(b) For the reaction that directly involves H2S donating a proton to C2H5NH2, calculate the corresponding equilibrium constant, K, at 25 °C.
(c) Using your result from part (b), justify which single reaction dominates the acid–base equilibrium of the system, and explain why other listed reactions contribute negligibly to the final pH.
(d) Perform a quantitative equilibrium calculation to determine the pH of the solution at 25 °C. Report the pH to two decimal places.
(e) Assess the validity of neglecting the second deprotonation step of H2S in your calculation. Support your answer with numerical evidence or a logical argument based on relative magnitudes of relevant equilibrium constants.