CBSE Class 12 Chemistry Redox Titration Worksheet

CBSE Class 12  Redox Titration. CBSE issues sample papers every year for students for class 12 board exams. Students should solve the CBSE issued sample papers to understand the pattern of the question paper which will come in class 12 board exams this year. The sample papers have been provided with marking scheme. It’s always recommended to practice as many CBSE sample papers as possible before the board examinations. Sample papers should be always practiced in examination condition at home or school and the student should show the answers to teachers for checking or compare with the answers provided. Students can download the sample papers in pdf format free and score better marks in examinations. Refer to other links too for latest sample papers. Click here to get important questions for class 12 chemistry chapter wise.

ESTIMATION OF \( \text{KMnO}_4 \) USING STANDARD MOHR’S SALT SOLUTION

Aim: To determine the mass of potassium permanganate in one litre of the given solution using the given pure Ferrous ammonium sulphate [Mohr’s salt, \( (\text{NH}_4)_2\text{SO}_4\cdot\text{FeSO}_4\cdot6\text{H}_2\text{O} \)] crystals of Analytical Reagent (A.R.) quality.

Principle: The estimation is based on the reaction between \( \text{KMnO}_4 \) and Mohr’s salt.

\[ \text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} \] \[ \{ \text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^- \} \times 5 \]

\[ \text{MnO}_4^- + 8\text{H}^+ + 5\text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} + 5\text{Fe}^{3+} \]

REQUIREMENTS: Burette, Pipette, Conical flask, Weighing bottle, Ferrous ammonium sulphate (Mohr’s salt) crystals (A.R. Grade), Balance, etc.

PROCEDURE: First of all wash all the apparatus thoroughly with tap water and rinse with distilled water.

Preparation of Mohr’s salt solution (standard 0.0500 M)

Weigh 1.960 g of Mohr’s salt crystals (A.R. grade), accurately in a weighing bottle transfer and wash into a clean funnel placed over a 100 ml. standard volumetric measuring flask. Add about 15ml (1 test tube) of 10% \( \text{H}_2\text{SO}_4 \) solution and then wash down the crystals carefully into the flask by a jet of distilled water. Wash the funnel also down into the flask. Dissolve the crystals completely and then make up the solution to 100 ml. mark. Shake well to make a homogeneous solution.
(Note: - Weigh 4.900 g of Mohr’s salt for a 250 ml. flask)

Mass of Mohr’s salt crystals = ---------------------- gms.
Volume to which the solution is diluted = \( \text{V}_s \) ml. = 100 ml.
Molarity of the Mohr’s salt solution = \( \text{M}_F = \frac{\text{a} \times 1000}{392 \times \text{V}_s} = \frac{\dots\dots\dots\dots\text{ x } 1000}{392 \text{ x } 100} = \dots\dots\dots\dots\text{M} \)

Estimation of Potassium permanganate solution

Pipette out 20 ml. of Mohr’s salt solution into a conical flask and add about 20 ml. (1 ½ test tubes) of Dil. Sulphuric acid to it. The solution is then titrated with \( \text{KMnO}_4 \) solution taken in the burette. The end point is indicated by the appearance of pale pink colour in the solution. The titration is repeated till concordant titre values are obtained.

Calculation: -

Molarity of the Mohr’s salt solution = \( \text{M}_F \) = ------------- M
Volume of FAS used = \( \text{V}_F \) = 20 ml
Volume of \( \text{KMnO}_4 \) used up = \( \text{V}_K \) ml. = --------------- ml.

By law of equivalence: - \( 5 \text{M}_K\text{V}_K = \text{M}_F\text{V}_F \)

Molarity of the \( \text{KMnO}_4 \) solution = \( \text{M}_K = \frac{\text{M}_F\text{V}_F}{5 \text{V}_K} = \frac{\dots\dots\dots\dots\text{ x } 20}{5 \text{ x }\dots\dots\dots\dots} = \dots\dots\dots\dots\text{M} \)

Result: -

Molarity of the given \( \text{KMnO}_4 \) solution = ------------- M.

N.B: - (Do not enter in your journal)

• 1. If during the reaction a brown precipitate of hydrated manganese dioxide is observed, insufficient sulphuric acid is indicated. More acid should then be added to the solution.
• 2. Since the lower meniscus cannot be seen clearly through the purple colour of the \( \text{KMnO}_4 \) solution, the burette readings are taken at the highest level of the liquid surface (upper meniscus).
• 3. A burette, which has been used for \( \text{KMnO}_4 \) solution, should be emptied and cleaned immediately after use. Any brown stain due to \( \text{MnO}_2 \) should be removed by oxalic acid solution or Mohr’s salt solution.

Experiment Number: 02

Date: ----------

ESTIMATION OF \( \text{KMnO}_4 \) USING STANDARD OXALIC ACID SOLUTION

Aim: To determine the mass of potassium permanganate in one litre of the given solution using the given pure oxalic acid crystals of Analytical Reagent (A.R.) quality.

Principle: The estimation is based on the reaction between \( \text{KMnO}_4 \) and Oxalic acid.

\[ \{ \text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} \} \times 2 \] \[ \{ \text{C}_2\text{O}_4^{2-} \rightarrow 2\text{CO}_2 + 2\text{e}^- \} \times 5 \]

\[ 2\text{MnO}_4^- + 16\text{H}^+ + 5\text{C}_2\text{O}_4^{2-} \rightarrow 2\text{Mn}^{2+} + 8\text{H}_2\text{O} + 10\text{CO2} \]

REQUIREMENTS: Burette, Pipette, Conical flask, Weighing bottle, Oxalic acid crystals (AR grade), Balance, etc.

PROCEDURE: First of all wash all the apparatus thoroughly with tap water and rinse with distilled water.

Preparation of oxalic acid solution (standard 0.0500 M)

Weigh 0.6300 g of Oxalic acid crystals (A.R. grade), accurately in a weighing bottle wash and transfer into a clean funnel placed over a 100 ml. standard volumetric measuring flask. Wash the funnel also down into the flask. Dissolve the crystals completely and then make up the solution to 100ml. mark. Shake well to make a homogeneous solution.
(Note: - Weigh 1.575 g of oxalic acid for a 250ml. flask)

Mass of oxalic acid crystals = X g
Volume of solution prepared = Vs ml = 100 ml.
Molarity of the Oxalic acid solution (Mo) = \( \frac{\text{X x } 1000}{126 \text{ x Vs}} = \frac{\dots\dots\dots\dots\text{ x } 1000}{126 \text{ x } 100} = \dots\dots\dots\dots\text{M} \)

Estimation of Potassium permanganate solution

Pipette out 20 ml. of oxalic acid into a conical flask, about 20 ml. (1 ½ test tubes) of Dil. Sulphuric acid is added and the mixture is heated to \( 60^\circ\text{C} \) to \( 70^\circ\text{C} \) (bearable warmth). The solution is then titrated with \( \text{KMnO}_4 \) solution taken in the burette. The end point is indicated by the appearance of pale pink colour in the solution. The titration is repeated till concordant titre values are obtained.

Calculation: -

Molarity of the Oxalic acid solution = Mo = ………………M
Volume of oxalic acid used = Vo = 20 ml
Volume of \( \text{KMnO}_4 \) used up = \( \text{V}_K \) ml. = ................ ml.

By law of equivalence: - \( 5 \text{M}_K\text{V}_K = 2 \text{M}_O\text{V}_O \)

Molarity of the \( \text{KMnO}_4 \) solution = \( \text{M}_K = \frac{2 \text{M}_O\text{V}_O}{5 \text{V}_K} = \frac{2 \text{ x }\dots\dots\dots\dots\text{ x } 20}{5 \text{ x }\dots\dots\dots\dots} = \dots\dots\dots\dots\text{ M} \)

Mass of \( \text{KMnO}_4 \) in one litre of the given solution = \( \text{M}_K \text{ x } 158 = \dots\dots\dots\dots\text{ x } 158 = \dots\dots\dots\dots\text{ g.} \)

Result: -

Molarity of the given \( \text{KMnO}_4 \) solution = ------------------- M.
Mass of \( \text{KMnO}_4 \) in one litre of the given solution = ------------------- g.

N.B: - (Do not enter in your journal)

1. If during the reaction a brown precipitate of hydrated manganese dioxide is observed, insufficient sulphuric acid is indicated. More acid should then be added to the solution.

2. Since the lower meniscus cannot be seen clearly through the purple colour of the \( \text{KMnO}_4 \) solution, the burette readings are taken at the highest level of the liquid surface (upper meniscus).

3. A burette, which has been used for \( \text{KMnO}_4 \) solution, should be emptied and cleaned immediately after use. Any brown stain due to \( \text{MnO}_2 \) should be removed by oxalic acid solution or Mohr’s salt solution.