Equilibrium - Result Question 95
####99. A physician wishes to prepare a buffer solution of $pH=3.58$ that efficiently resists a change in $pH$ yet contains only small concentrations of the buffering agents. Which one of the following weak acids together with its sodium salt would be the best to use ?
[1997]
(a) $m$-chlorobenzoic acid $(pK_a=3.98)$
(b) $p$-chlorocinnamic acid $(pK_a=4.41)$
(c) 2,5-dihydroxy benzoic acid $(pK_a=2.97)$
(d) Acetoacetic acid $(pK_a=3.58)$
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Solution:
(d) $pH=p K_a+\log \frac{[\text{ Salt }]}{[\text{ Acid }]}$
For acetoacetic acid
$3.58=3.58+\log \frac{[\text{ Salt }]}{[\text{ Acid }]}$
$\Rightarrow \log \frac{[\text{ Salt }]}{[\text{ Acid }]}=0$
$\Rightarrow \frac{[\text{ Salt }]}{[\text{ Acid }]}=1$
This is the required condition for maximum buffer capacity.
