Question

Aniline is an amine that is used to manufacture dyes. It is isolated as aniline hydrochloride $\left[C_6{H}_5{NH}_3\right]Cl$, a salt of aniline and $HCl$. Calculate the pH of $0.233M$ solution of aniline.
$K_b\left(aniline\right)=4.6\times {10}^{-10}$

• A. $3.93$
• B. $6.94$
• C. $2.64$
• D. none of these

Answer 1

The correct option is C $2.64$

Aniline hydrochloride $(C_6{H}_{5}N{H}_2.HCl)$ is conjugate acid of aniline. To calculate $pH$ we have to find $p{K}_a$ of $C_6{H}_{5}N{H}_2.HCl$ and treat it like weak acid.

We know that $p{K}_a+p{K}_b=p{K}_w$

$\Rightarrow p{K}_a=14-(-\log 4.6\times {10}^{-10})=4.66$

$\therefore pH=\frac{1}{2}p{K}_a-\frac{1}{2}\log C$

$=\frac{4.66}{2}+\frac{0.63}{2}=2.64$