Question

The best indicator for titrating HCl with $N{{H}_{4}}OH$ is
A. Litmus
B. Phenolphthalein
C. Phenol red
D. Methyl Orange

Answer 1

Titration refers to determination of concentration of a solution with respect to water with a pH of 7. In acid – base titration indicators are chemical reagents which are used to indicate the end point. The end point or equivalence point in an acid base titration is the point where the number of moles of acid and base are same.

Complete answer:
The best indicator for titrating $HCl$ with $N{{H}_{4}}OH$ is –
Let us first understand the characteristics of indicators. Indicators are generally used to detect the end point of titration. They change color in response to the chemical change occurring. They are usually weak acid or bases but their conjugate base or acid forms have different color due to difference in the absorption spectra. There are many different indicators used for various acid base titrations.
Here, in the question $HCl$ which is a strong acid is titrated against $N{{H}_{4}}OH$ which is a weak base. The reaction between the two leads to formation of $N{{H}_{4}}Cl$ salt. The reaction involved is-
$HCl+N{{H}_{4}}OH\to N{{H}_{4}}Cl+{{H}_{2}}O$
Due to the hydrolysis of the salt, the pH lies in the acid range. pH at the end point lies between 6 to 4. The methyl orange is best suited for the purpose.

Thus, option (D) is the correct option.

Note: Students must understand that in order to choose a suitable indicator, it is necessary to understand the pH changes in the titration. The change in pH in the vicinity of the equivalence point is most important. Thus, a single indicator cannot determine the equivalence point of all different titration systems. Phenolphthalein is chosen for a strong acid – a strong base titration while methyl orange is suited for strong acid – weak base system.