The table gives base-dissociation constants measured at 25 °C for three aqueous…

CHEMISTRY · Multi-part · A Level · Chemistry · Amines · Amines

Problem

The table gives base-dissociation constants measured at 25 °C for three aqueous amines of equal molarity.
BaseKbK_b
CH3_3NH2_24.4×10−44.4\times10^{-4}
(CH3_3)2_2NH6.3×10−46.3\times10^{-4}
C2_2H5_5NH2_25.6×10−45.6\times10^{-4}
Use Kw=1.0×10−14K_w=1.0\times10^{-14}. (1) Calculate pKapK_a for the conjugate acid of each base. Show your work clearly. (2) Rank the three conjugate acids from strongest to weakest acid. (3) Explain the ranking in part (b) by referring to inductive or electron-donating effects of the alkyl groups on nitrogen. (4) A fourth base, (CH3_3)3_3N, has Kb=6.4×10−5K_b=6.4\times10^{-5}. Predict where its conjugate acid would appear in the ranking established in part (b) and justify your prediction.

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