To prepare M/20 solution of ferrous ammonium sulphate(Mohr’s salt).with its help, determine the molarity and strength of the given solution of potassium permanganate (KMnO4).

 

Aim- To prepare M/20 solution of ferrous ammonium sulphate (Mohr’s salt).with its help, determine the molarity and strength of the given solution of potassium permanganate (KMnO₄).

Apparatus required- Chemical balance, weight box, conical flask, burette, Burette stand, 100 ml Beaker, 20 ml pipette, funnel, wire gauge, wash bottle and burner.

Chemical required- potassium permanganate, dilute sulphuric acid, distilled water and ferrous ammonium sulphate.

Theory-

      ·   Ferrous ammonium sulphate is a double salt and act as reducing agent against KMnO₄

      ·   Whereas potassium permanganate act as an oxidising agent and it oxidises iron present in Mohr’s salt. in the acidic medium KMnO₄ behave as a very strong oxidizing agent. So acidity is introduced by adding dil. H₂SO₄.

     ·   Hence the reaction between the titrant (KMnO₄) and the analyte (Mohr’s salt) is a redox reaction, Therefore the titration is called redox reaction.

     ·   Potassium permanganate act as a self-indicator for this reaction.   

     ·  The colour at the end point changes from colourless to light pink. The endpoint of a titration is the point at which the reaction between the titrant and the analyte becomes complete.

 Chemical Reaction involved-

(a)Molecular equation-

Reduction half reaction –

2KMnO₄   + 3H₂ SO₄ ------>   K₂SO₄ + 2MnSO₄   +   3H₂O +   5[O] ---(1)

when dil. Sulphuric acid is added with KMnO₄, it produces [O]nascent oxygen.

·  Oxidation half reaction –[O] oxidises the Mohr’s salt

[   FeSO₄(NH₄)₂SO₄.6H₂O + 3H₂ SO₄   +  [O]---->  Fe₂ (SO₄)₃ + 2(NH₄)₂SO₄  +  13H₂O ] x5  ---(2)

by combining of eq. (1) and (2)

2KMnO₄   + 3H₂ SO₄  + 5x2FeSO₄(NH₄)₂SO₄.6H₂O +  5x3H₂ SO₄ + 5[O] ------------> 

K₂SO₄  + 2MnSO₄   +  3H₂O +  5[O] +  5Fe₂ (SO₄)₃+ 5x2(NH₄)₂SO₄  + 5x13H₂O

Then, Net reaction may be written as(Overall reaction) –

2KMnO₄   + 3H₂ SO₄  +  5x2FeSO₄(NH₄)₂SO₄.6H₂O +  5x3H₂ SO₄  -----> K₂SO₄   +   2MnSO₄   + 5Fe₂ (SO₄)₃ + 10(NH₄)₂SO₄    +  68H₂O  ……(3)

(b) Ionic equation-

MnO₄^-   + 8H⁺ +  5e^-    ------>   Mn²⁺ +  4H₂O  ……..(4)

[Fe²⁺ -------->   Fe²⁺  +  e^- ] x5  .....….(5)

by combining of equation (3) and (4), Net ionic equation ma be written as-

MnO₄^-   + 8H⁺    +   5Fe²⁺  +   5e^-    -------->  5Fe²⁺  +  Mn²⁺ +  4H₂O  +  5e^- ………..(6)

Procedure-

     ·       4.9g of Mohr’s salt crystal are weighed and dissolved in 250ml of water in a measuring flask to prepare M/20 solution of it.

     ·       with the help of pipette, 20 ml Mohr’s salt is taken out in titration flask and one test tube (10ml) full of dilute H₂SO₄ is added to it. Since it makes acidic medium to the reaction and prevents Mohr’ salt from hydrolysis.

     ·       Burette is washed with distilled water and is filled with KMnO₄ solution given and its initial reading is noted.

     ·       Now we start adding KMnO₄ solution from the burette drop wise into titration conical flask already taken 20 ml Mohr’s salt and one full test tube of H₂SO₄ in it with continuous swirling until a permanent light pink just appear in the solution of titration conical flask.

     ·     Note down the reading of burette at which permanent light pink colour appeared in conical flask.

·       Thus this process repeated 3-times to obtained three concordant reading.

Observation-

      ·       Molecular mass of Mohr’s salt =392g/mol

      ·       Molecular mass of KMnO₄ = 158g/mol

      ·       Molarity of the Mohr’s salt solution=M/20 = 1/20 mol/litre

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

Observation table-

s.no	Volume of Mohr’s salt solution taken by Pipette in ml (V1)	Volume of unknown KMnO4 solution taken by burette in (ml)			Concordant volume of KMnO4 used in ml (V2)
		Initial	Final	Volume of KMnO4 sol used
1.	20ml	0.0	9.5	9.5	9.43
2	20ml	0.0	9.4	9.4
3	20ml	0.0	9.4	9.4

                                            

Calculation-

Molarity of KmnO4 solution from the balanced ionic Chemical equation given in theory. it clear that 1mol  KMnO4 reacts with 5mol FeSO₄(NH₄)₂SO₄.6H₂O (according to molarity equation)

Therefore,        

 FeSO₄(NH₄)₂SO₄.6H₂O(Mohr’s salt)  Vs      KMnO₄            

  n₂ M₁V₁        =   n₁M₂V₂

  1 M₁V₁      =   5 M₂V₂               

 So,

 M₂  =  M₁V₁/5V₂                                 

Here-

[ n₁ = no. of mole of Mohr’s salt = 5mol     M₁= no. of mole of Mohr’s salt

V_{1 =}volume of Mohr’s salt

n₂ = no. of mole of KMnO₄   = 1mol                                                                                                    

M₂= no. of mole of KMnO₄ =?

V₂=volume of KMnO₄                                      

 So,      M₂  =  M₁V₁/5V₂  

by putting the value of M_1, V₁ and V₂ in above formula 

 M ₂     =  1 x 20 /20 x 5 x 9.43       

 M ₂ =  0.0212M                                                                          

Thus the molarity of KMnO₄ ( M ₂)   = 0.0212 M 

then, the strength of    KMnO₄   will be                                                                   

 = molarity of KMnO₄(M₂) x Molecular mass of KMnO₄

= 0.0212 x 158 gm/litre

= 3.349 gm/litre

Result – We found that molarity and strength of KMnO₄ as 0.0212M and 3.349gm/litre respectively.

Precautions –

     ·       We should Always add one test tube(10ml) dilute sulphuric acid before titration.

     ·        KMnO₄ should be added drop wise other wise the solution will become brown due to MnO₂ form.

     ·       We should always take reading KMnO₄ solution in the burette from the lower meniscus.

·       Should always use distilled water for preparing solution and washing chemical apparatus.