Ethylenediamine, H NCH CH NH (abbreviated "en")

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

Ethylenediamine, H2_2NCH2_2CH2_2NH2_2 (abbreviated "en"), is a diprotic base that can accept two protons in separate steps. • Kb1=5.6×10−4K_{b1}=5.6\times10^{-4} (for the first proton-accepting site) • Kb2=1.3×10−8K_{b2}=1.3\times10^{-8} (for the second proton-accepting site) A 50.0 mL50.0\,\text{mL} sample of 0.0500 M0.0500\,\text{M} ethylenediamine is titrated with 0.100 M0.100\,\text{M} nitric acid. Consider the point in the titration at which exactly one proton per ethylenediamine molecule has been transferred (the first equivalence point). Parts: (a) Calculate the volume of nitric acid required to reach the first equivalence point. (b) Determine the molar concentration of the singly protonated ethylenediamine species in the mixture immediately after the first equivalence point is reached. (c) Compute the pH of the solution at this first equivalence point, clearly outlining your reasoning. (d) Discuss briefly why the second base-dissociation constant Kb2K_{b2} has negligible influence on the pH determined in part (c).

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