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Prepare an aqueous buffer solution of $\mathrm{pH}=2.85$ that efficiently resists a change in pH yet contains only a small amount of buffering agent. Which one of the following weak acid together with its salt would be best to use?

A) m-chlorobenzoic acid ($\mathrm{p}K_a=3.98$)

B) p-chloro cinnamic acid ($\mathrm{p}K_a=4.41$)

C) 2,5-dihydroxy benzoic acid ($\mathrm{p}K_a=2.97$)

D) Acetoacetic acid ($\mathrm{p}K_a=3.58$)

Looking at the $\mathrm{p}K_a$ values C) is the strongest acid. However I cannot get if the strongest acid is best suited for buffer action which resists change in pH when small amounts of acid and base are added to it.

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closed as off-topic by Jan, Mithoron, Tyberius, Jon Custer, airhuff Nov 8 '17 at 17:32

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It is not the compound that has the lowest pKa value that is important, but which one has their pKa value closest to the desired buffer's pH. If I wanted a buffer at pH 9, I wouldn't use any of these.

What happens to the distribution of the acid and salt as the pH moves away from the pKa value?

What distribution of acid and salt makes the best buffer?

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  • $\begingroup$ Answering with counter-questions is basically screaming for an NAA flag. $\endgroup$ – Jan Nov 8 '17 at 8:10
  • $\begingroup$ When $pH$ moves away from $pK_a$ value the concentration of acid and salt vary more.But I couldn't get how this distribution matters for making the best buffer?Thanks. $\endgroup$ – user471651 Nov 8 '17 at 8:30
  • $\begingroup$ @Jan The questions guide him to the answer. $\endgroup$ – LDC3 Nov 9 '17 at 14:12

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