Questions & Answers

In the titration of Mohr salt against $KMn{{O}_{4}}$ the indicator used is:
(a)- Diphenylamine
(b)- $KMn{{O}_{4}}$
(c)- Phenolphthalein
(d)- Methyl orange.

Answer Verified Verified
Hint: The titration between Mohr's salt and potassium permanganate is a redox titration. This titration is an example of self indicator titration. The endpoint in the reaction is colorless to permanent light pink.

Complete answer:
An indicator is a substance that responds to the change in a chemical reaction by a change in color.
It is used in the acid-base titration. The change in the pH of the solution will change the color of the indicator.
In a redox reaction, the change in a chemical reaction is indicated by a change in color of the indicator.
The indicator is added in the solution before the titration, the change in color is noted as the endpoint.
 In the titration of mohr salt against $KMn{{O}_{4}}$, $KMn{{O}_{4}}$ acts as the indicator.
The titration of mohr salt against $KMn{{O}_{4}}$ is based upon oxidation-reduction titration.
Mohr salt is known as ferrous ammonium sulfate.
The ferrous ammonium sulfate is titrated against potassium permanganate in the presence of sulfuric acid in an acidic medium. Precipitation of manganese oxide will occur if the titration will not take place in an acidic medium. Here, $KMn{{O}_{4}}$ will act as a self indicator because of the very intense purple color. When the solution changes from colorless to permanent pink color the endpoint is noted.
So, the correct answer is “Option B”.

Note: In this redox reaction, ferrous ion from Mohr’s salt gets oxidized, and pink colored manganese present in potassium permanganate, and the manganese gets reduced to colorless.

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