Class 12 Record Experiment 1

 (WRITE IN RHS)                                 EXPERIMENT 1                                           DATE : 

 Estimation of unknown KMnO4 using prepared standard M/20 Mohr s salt solution

AIM:-

1. To find the mass of Mohrs salt to prepare 250ml of  (M/20) solution.

2. To find the molarity of unknown KMnO4 solution using M/20 Mohr s salt solution.

3. To estimate the strength of KMnO4 as grams in 1 litre of KMnO4 solution.

THEORY:-

Mohr s salt a primary standard is chemically FeSO4(NH4)2SO46H2O (molar mass = 392g/mol)

Oxidising agent: unknown potassium permanganate solution

Reducing agent: standard Mohr s salt solution (FeSO4(NH4)2SO4 .6H2O)

To prepare M/20 solution of Mohr s salt, the weight of Mohr s salt required can be calculated by using the formula

Molarity =    (WbX1000) / (Mb X 250)    

          Wb = (Molarity X Mb) / 4   

             =  (0.05 X 392) / 4      =  4 .9 g

Here 

Mb = Molecular mass of Mohr s salt              = 392g/mol

Wb = Weight of Mohr s salt required             =4.9g

KMnO4 (molar mass = 158g/mol) is an oxidising agent and it oxidises iron (Fe) present in Mohr s salt from +2 to +3 oxidation state.

The manganese (Mn) of KMnO4 is reduced from +7 to +2 oxidation state.

Since both oxidation and reduction happens simultaneously it is called a redox reaction and the titration is called redox titration. 

KMnO4 acts as a self-indicator and the colour at the end point changes from colourless to pink.

(WRITE IN LHS) OBSERVATIONS AND CALCULATIONS: -  

AIM 1:-

Weight of watch glass                                                                    = 10g

Weight of watch glass + Mohr s salt                                            = 14.9g

Weight of Mohr s salt dissolved in 250ml of distilled water = 4.9g

Molarity of Mohr s salt solution prepared                                 = (4.9 × 4)/392 =M/20

AIM 2 and 3 :-

Volume of Mohr s salt solution           (V1)                                     = 20ml 

Molarity of Mohr s salt solution         (M1)                                     = (M/20)

Standard Mohr s salt solution (M/20)  against  unknown KMnO4 solution SELF INDICATOR 

SL.NO.	VOLUME OF M/20 MOHR S SALT SOLUTION	INITIAL  READING OF BURETTE	FINAL READING  OF BURETTE	VOLUME OF  KMnO4 USED
1.	....20......ml	..........ml	..........ml	..........ml
2.	....20......ml	..........ml	..........ml	..........ml
3.	....20......ml	..........ml	..........ml	..........ml

PRINCIPLE OF VOLUMETRIC ANALYSIS

  (M1V1) / n1 = (M2V2) / n2

From ionic equations:- 

n1  = stoichiometric coefficient of reactant 1 Mohr s salt =5

n2  = stoichiometric coefficient of reactant 2 Potassium permanganate =1

Volume of potassium permanganate solution  = (V2)   = ...........ml

Therefore Molarity of KMnO4  solution                 = (M2) = (M1V1n2)/(n1V2)

                                                                                                = ((1/20)X20X1)/( 5V2 )  = 1/(5V2)  =  ------M 

Molecular mass of potassium permanganate     = 158g/mol

Strength of potassium permanganate M2 X 158 = ...............M × 158g/mol   = ................g/l

CONCLUSIONS:-

1. Mass of Mohr s salt used to prepare M/20 , 250ml solution = .........g

2.Molarity of the given KMnO4 solution = ............M

3. Strength of given KMnO4 solution = ............g/l

(WRITE IN RHS)

PROCEDURE: - 

1) Preparation of 250ml standard solution of M/20 mohr s salt

4.9 g of Mohr s salt is weighed into a 250ml standard flask, added 5-10 ml of dilute sulphuric acid to prevent hydrolysis and made up to the mark using distilled water and labelled as M/20 Mohr s salt solution.

2) Titration and estimation of unknown KMnO4 with standard M/20 mohr s salt solution.(learn well all the steps of procedure for exam) 

1. Take 50ml burette and wash it with distilled water and then rinse it with few ml of given KMnO4 solution and place it in the burette stand after filling with unknown KMnO4 with the help of a funnel.

2. Take a pipette and wash it with distilled water and then rinse it with few ml of given standard (M/20) Mohr s salt solution. Pipette out 20ml into a clean 250ml conical flask. 

3. Add 20ml (Approximately one test tube full) of dilute sulphuric acid into the conical flask to make the solution acidic.

4. Place the conical flask along with its content below the nozzle of the burette and start adding KMnO4 solution from the burette drop wise into the conical flask with continuous swirling until a permanent pink colour just appear in the solution of conical flask.

5. Note down the reading of the upper meniscus of the KMnO4 solution in the burette and repeat the experiment to get three concordant values ( close values)

BURETTE SOLUTION	UNKNOWN POTASSIUM PERMANGANATE  SOLUTION
PIPETTE SOLUTION	M/20  MOHR S SALT SOLUTION  FeSO4(NH4)2SO46H2O
(SELF) INDICATOR	POTASSIUM PERMANGANATE KMnO4
ENDPOINT	COLOURLESS TO PERMANENT LIGHT PINK

RESULT:-

1. Mass of Mohr s salt used to prepare M/20 , 250ml solution = .........g

2.Molarity of the given KMnO4 solution = ............M

3. Strength of given KMnO4 solution = ............g/l

PRCAUTIONS:-

1. Always add one test tube full of 4N sulphuric acid to 20ml of Mohr s salt solution before titration.

2. Always add KMnO4 solution drop wise otherwise the solution will become brown due to MnO2 formation.

3. Always add dilute sulphuric acid into Mohr s salt solution while preparing.

4. Always take the reading of KMnO4 solution in the burette from the upper meniscus.

5. Always use distilled water for preparing the solution.