Question

Indicator	pH range
Methyl violet	0.3-1.8
Methyl orange	2.8-3.8
Congo red	2.8-4.8
Methyl red	3.8-6.1
Bromothylmol blue	6.0-7.9
Phenol Red	6.8-8.6

The table above shows the pH ranges of some common indicators. Given that $K_a$ is $7.3\times {10}^{-10}$ for boric acid $\left(H_{3}B{O}_3\right)$, Choose an indicator that can be used for the titration of $0.10MK{H}_{2}B{O}_3$ with 0.10 M HCl.

• A. Methyl orange
• B. Congo Red
• C. Methyl Red
• D. Phenol Red

Answer 1

The correct option is C Methyl Red

$K{H}_{2}B{O}_3+HCl\to H_{3}B{O}_3+KCl$.

At equivalent point the concentration of $H_{3}B{O}_{3}produced=0.050M{H}_{3}B{O}_3$ then ionizes thus :

$H_{3}B{O}_3+H2O\to H_{3}B{O}_3+H_3{O}^{+}$

0.05 M $x$ $x$

$k_a=\frac{\left[H_3{O}^{+}\right]\left[H_{3}B{O}_3\right]}{\left[H_{3}B{O}_3\right]}=\frac{x^2}{0.05}$

$i.e.7.3\times {10}^{-10}=x^2/0.05$

$x=6.04\times {10}^6$

$pH=-log(6.04\times {10}^6)=5.22$

This pH is in the range of methyl red indicator which will be suitable for the titration.

Hence the correct option is C.