Important Compounds of Chromium and Manganese - K2Cr2O7 & KMnO4 - UNSOLVED PRACTICE SET

Q7. Why does the colour of K₂Cr₂O₇ solution change from orange to yellow on adding NaOH? Write the reaction involved.

Q8. Write the balanced ionic equation for the reaction of KMnO₄ with FeSO₄ in acidic medium.

Q9. Why is KMnO₄ solution not used as a primary standard in volumetric analysis? What precaution is taken while preparing its standard solution?

Q10. What happens when K₂Cr₂O₇ is heated with concentrated H₂SO₄ and NaCl? Write the reaction.

Q11. Why does KMnO₄ act as a stronger oxidizing agent in acidic medium than in alkaline or neutral medium?

Q12. Your chemistry teacher uses KMnO₄ to treat a student's minor wound in the lab. Why is KMnO₄ used as an antiseptic? What property makes it suitable?

Q13. (a) Describe the preparation of potassium permanganate (KMnO₄) from pyrolusite ore (MnO₂).

(b) Write the balanced chemical equations involved.

(c) Why is KMnO₄ stored in dark bottles?

(d) What happens when KMnO₄ is heated? Write the reaction and explain the products.

Q14. (a) Describe the preparation of potassium dichromate (K₂Cr₂O₇) from chromite ore (FeCr₂O₄).

(b) Write the balanced chemical equations for the steps involved.

(c) Explain why K₂Cr₂O₇ is preferred over Na₂Cr₂O₇ as a primary standard in volumetric analysis.

(d) What happens when K₂Cr₂O₇ is heated? Write the reaction.

Q15. (a) Compare the oxidizing power of KMnO₄ in acidic, neutral, and alkaline media. Write the reduction products in each case.

(b) A student is given a solution that may contain Fe²⁺ or Fe³⁺. How can KMnO₄ be used to determine which ion is present?

(c) Why is K₂Cr₂O₇ used in breath analysers to detect alcohol in drivers? Write the reaction involved.

Q16. In a redox titration, 25.0 mL of 0.1 M FeSO₄ solution is titrated against KMnO₄ solution in acidic medium.

(a) Write the balanced redox reaction.

(b) Calculate the volume of 0.02 M KMnO₄ required to reach the endpoint.

(c) What colour change indicates the endpoint? Why is no external indicator needed?

(d) If the KMnO₄ solution was prepared 2 weeks ago and had partially decomposed, would the calculated concentration of FeSO₄ be higher or lower than actual? Explain.

(e) Why is the titration carried out in acidic medium (H₂SO₄) and not in HCl or HNO₃?

Q17. The following experiments are performed in the laboratory:

Experiment 1: 20 mL of oxalic acid solution (H₂C₂O₄) requires 16 mL of 0.05 M KMnO₄ in an acidic medium for complete oxidation.

Experiment 2: 25 mL of ferrous ammonium sulphate (Mohr's salt) solution requires 20 mL of the same KMnO₄ solution.

(a) Write the balanced redox reactions for both experiments.

(b) Calculate the molarity of the oxalic acid solution.

(c) Calculate the molarity of Mohr's salt solution.

(d) Calculate the mass of Mohr's salt (FeSO₄·(NH₄)₂SO₄·6H₂O, molar mass = 392 g/mol) present in 1 litre of the solution.

(e) Why is oxalic acid solution heated to 50-60°C before titration with KMnO₄, but Mohr's salt solution is titrated at room temperature?

Q18. In a school chemistry lab, students perform various tests using K₂Cr₂O₇ and KMnO₄.

(a) A student adds K₂Cr₂O₇ solution to ethanol and warms the mixture. The orange solution turns green. Write the reaction and identify the oxidation and reduction half-reactions.

(b) Another student uses KMnO₄ to test for the presence of SO₂ gas. The purple colour of KMnO₄ disappears. Write the reaction and explain the colour change.

(c) A third student prepares chromyl chloride (CrO₂Cl₂) by heating K₂Cr₂O₇ with NaCl and concentrated H₂SO₄. The red vapours are passed through NaOH solution, forming a yellow solution. Identify the yellow compound and write the reactions involved.

(d) The students learn that KMnO₄ is used in water treatment plants. Why is it added to drinking water? What concentration is typically used and why must it be carefully controlled?

(e) A teacher demonstrates the "chemical chameleon" experiment by adding sugar to alkaline KMnO₄. The solution changes from purple to green to brown. Explain the sequence of colour changes with the reactions involved.