Question

The range of most suitable Indicator which should be used for titration of ${X^ - }N{a^ + }$ $\left( {0.1M,\;10mL} \right)$ with $0.1\;M$ $HCl\;$ should be:
(Given that ${{\text{K}}_{{\text{b}}\left( {{{\text{X}}^ - }} \right)}} = {10^{ - 6}}$ )
(A) $2 - 3$
(B) $3 - 5$
(C) $6 - 8$
(D) $8 - 10$

Answer 1

We are solving this question which is related to titration so, we should know what titration is. Titration is a chemical technique where we use a solution whose concentration is known to us to find the concentration of another solution. The solution whose concentration is known to us is known as ‘titrant’ and the other solution is known as ‘analyte’. In this question we are doing an Acid – Base titration in which we use an acid or base of known quantity to neutralize the unknown solution.

Complete answer:
We are given ${X^ - }N{a^ + }$ which is a monatomic (single atom) weak base, we will neutralize it with $HCl\;$ which is a strong acid.
Reaction taking place will be:
$NaX + HCl \to NaCl + HX$
we will calculate the $pH\;$ by following formula:
$pH = \dfrac{1}{2}\left[ {p{K_{\text{w}}}\; - \;p{K_{{\text{b}}\;}} - \;logC} \right]$
where ${K_{\text{w}}}$ is the Ionic Product, ${K_{{\text{b}}\;}}$ is the base dissociation constant (given), and C is the Concentration.
Ionic product, ${{\text{K}}_{\text{w}}} = 1.0 \times {10^{ - 14}}$
Base dissociation Constant, ${{\text{K}}_{\text{b}}} = {10^{ - 6}}$
Concentration, ${\text{C}} = 0.1{\text{M}}$
Now, we put all the values in the $pH\;$ formula to get the $pH\;$
$pH = \dfrac{1}{2}\left[ {14\; - \;6 - \;log(0.1)} \right]$
$pH = 4.5$
So, option (B) is correct as the $pH\;$ lies between $3 - 5$ .

Additional Information:
We calculated the range of $pH\;$ for an indicator that should be suitable for this titration. We should know what indicators are. Indicators are those whose solution changes colour due to change in $pH\;$ , they usually have weak acids or weak bases. These indicators help us in identifying the nature of the solution whether it is acidic or basic. Some of the common indicators used are Phenolphthalein and Methyl red.

Note:
$pH$ is known as ‘potential of hydrogen’ or ‘power of hydrogen’ it is used to identify acidity or basicity of a solution The ionic product ${{\text{K}}_{\text{w}}}$ is calculated by the product of concentration of ${{\text{H}}^ + }$ ions and concentration of $O{H^ - }$ ions.