Redox titrations with potassium permanganate — NEET & JEE Questions

Question 1 · difficulty L1 · recall

Why is potassium permanganate described as a self-indicator in many redox titrations?

• A. It produces a fluorescent precipitate at equivalence
• B. It becomes colourless before any reaction begins
• C. It releases a separate acid-base indicator
• D. A slight excess gives a persistent colour

Question 2 · difficulty L2 · understanding

Which acid is preferred for acidifying a potassium permanganate redox titration?

• A. Dilute sulphuric acid
• B. Concentrated hydrochloric acid
• C. Concentrated nitric acid
• D. Aqueous acetic acid as the universal choice

Question 3 · difficulty L2 · understanding

Potassium permanganate titrations are carried out in the presence of dilute sulphuric acid rather than hydrochloric or nitric acid. Why is hydrochloric acid unsuitable?

• A. It is too weak an acid
• B. It reacts with oxalic acid
• C. It is coloured
• D. Chloride ions are oxidised by KMnO$_4$ to chlorine, consuming extra permanganate and giving a wrong titre

Question 4 · difficulty L2 · understanding

In the titration of oxalic acid with KMnO$_4$, no external indicator is added. The end point is detected when

• A. the first permanent light pink colour appears in the flask, because KMnO$_4$ acts as a self-indicator
• B. the solution turns from pink to colourless
• C. a brown precipitate of MnO$_2$ appears
• D. gas evolution stops

Question 5 · difficulty L3 · application

In strongly acidic solution, permanganate ion is reduced mainly to which manganese species?

• A. MnO2
• B. Mn2+
• C. MnO4^2−
• D. Mn metal

Question 6 · difficulty L3 · application

In acidic medium the reaction is 2MnO$_4^{-}$ + 5C$_2$O$_4^{2-}$ + 16H$^{+}$ $\rightarrow$ 2Mn$^{2+}$ + 10CO$_2$ + 8H$_2$O. 20.0 mL of 0.100 M oxalic acid requires what volume of 0.050 M KMnO$_4$?

• A. 40.0 mL
• B. 16.0 mL
• C. 10.0 mL
• D. 8.0 mL

Question 7 · difficulty L3 · application

25.0 mL of 0.100 M Mohr's salt (ferrous ammonium sulphate) solution is titrated with KMnO$_4$ in acidic medium: MnO$_4^{-}$ + 5Fe$^{2+}$ + 8H$^{+}$ $\rightarrow$ Mn$^{2+}$ + 5Fe$^{3+}$ + 4H$_2$O. If 25.0 mL of the permanganate solution is used, its molarity is

• A. 0.100 M
• B. 0.500 M
• C. 0.020 M
• D. 0.050 M

Question 8 · difficulty L3 · application

The oxalic acid-permanganate titration is performed after warming the oxalic acid solution to about 50-60 $^\circ$C, but the Mohr's salt-permanganate titration is done cold. Which pair of reasons is correct?

• A. Oxalic acid is insoluble in cold water; Mohr's salt decomposes on heating
• B. Warming increases the colour of KMnO$_4$; Fe$^{2+}$ is colourless
• C. Both should actually be heated; the Mohr's salt is kept cold only for convenience
• D. At room temperature the oxalate-permanganate reaction is slow (it is catalysed by the Mn$^{2+}$ formed), so warming is needed; Fe$^{2+}$ would be oxidised by air on heating, so the Mohr's salt solution is not heated

Question 9 · difficulty L4 · analysis

An oxalate-permanganate titration is very slow at room temperature but becomes faster after gentle warming. What explains this?

• A. Oxalate cannot exist below its boiling point
• B. Warming converts permanganate into an acid-base indicator
• C. Warming helps overcome the reaction's activation barrier
• D. Heating prevents all manganese ions from forming

Question 10 · difficulty L4 · application

In acidic medium, MnO$_4^{-}$ + 8H$^{+}$ + 5e$^{-}$ $\rightarrow$ Mn$^{2+}$ + 4H$_2$O. Given the molar masses KMnO$_4$ = 158 g mol$^{-1}$ and oxalic acid dihydrate = 126 g mol$^{-1}$, what are the equivalent masses of KMnO$_4$ (as oxidant) and of oxalic acid dihydrate (as reductant) in this titration?

• A. 31.6 and 63
• B. 158 and 126
• C. 52.7 and 42
• D. 79 and 63