Question

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

Give $p{K}_a$ of indicator $=5$

This $p{K}_a$ value corresponds to methyl red indicator. Methyl red is a suitable indicator for titrating carbonates of weak acids (ex. $H_{2}C{O}_3$ salt $N{a}_{2}C{O}_3$) with strong acid $HCl$.

Above titration reaction complated in following steps.

a) $N{a}_{2}C{O}_3\left(aq\right)+HCl\left(aq\right)\to NaHC{O}_3\left(aq\right)+NaCl\left(aq\right)$

b) $NaHC{O}_3\left(aq\right)+HCl\left(aq\right)\to NaCl\left(aq\right)+C{O}_2\left(g\right)+H_{2}O\left(aq\right)$

Correct option (C)