= Solution
The acceptor-strength trend is \b[<amide> <oxygen> and <amine> <nitrogen> strongest, <ketone> <oxygen> weaker, <hydrocarbon> essentially nonaccepting]. <trimethylamine> and <isopropylamine> each provide a relatively available <nitrogen> <lone pair>; adding N–H bonds changes donor ability much more conspicuously than it changes the acceptor ranking in this comparison. Alkyl substitution and <solvation> modify the precise values, so the matching <amine> parameters should be read as the stated empirical strengths rather than a universal equality of all <amines>.
In <acetamide>, <nitrogen> lone-pair donation contributes an $\mathrm{O^-{-}C{=}N^+}$ form contributing to <resonance in chemistry>, increasing <electron density> at <oxygen> and making it a strong <hydrogen bond acceptor>. \b[The <amide> acceptor is <oxygen>, not <nitrogen>]: the <nitrogen> <lone pair> is delocalized into the <carbonyl>. <acetone> also accepts at <oxygen>, but lacks this <nitrogen>-to-<carbonyl> donation and has the smaller <hydrogen-bond acceptor parameter>. <isobutane> has neither a heteroatom <lone pair> nor a comparable acceptor site; its zero value is consistent with the absence of an ordinary directional <hydrogen bond acceptor>.
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