Question

A certain indicator (an organic dye) has $p{K}_a=5$ for which of the following titrations may it be suitable.

• A. Acetic acid against NaOH
• B. Aniline hydrochloride against NaOH
• C. Sodium carbonate against HCl
• D. Barium hydroxide against oxalic acid

Answer 1

The correct option is C Sodium carbonate against HCl

$pH$ range of an indicator = $pKa-1$ to $pKa+1$

$p{K}_{HIn}=5$
Hence for the given indicator , color change occurs if $pH$ jumps from 4 to 6.

For the titration in order to show color change, $pH$ jump matches this range at the equivalence point.
(a) $C{H}_{3}COOH+NaOH,$ end point pH > 7
(b) Aniline hydrochloride + NaOH end point pH > 7
(c) $N{a}_{2}C{O}_3+HCl\to$ end point pH < 7
(d) $Ba(OH{)}_2+H_2{C}_2{O}_4,$ end point pH > 7
Hence, for $P{k}_a=5,$ titration of $N{a}_{2}C{O}_3$ against HCl is suitable.