AIM – (a) To prepare 250ml of M/20 solution of Mohr’s salt.
(b) Using this calculate the molarity and strength of the given KMnO4 solution.
APPARATUS AND CHEMICALS REQUIRED- Mohr’s salt, weighing bottle, weight box, volumetric flask, funnel, distilled water, chemical balance, dilute H2SO4, beakers, conical flask, funnel, burette, pipette, clamp stand, tile, KMnO4 solution.
THEORY- (a) Mohr’s salt having the formula FeSO4.(NH4)2SO4.6H2O has molar mass 392gmol-1. It is a primary standard.
Its equivalent mass is 392/1 = 392 as its n factor is 1 as per the following reaction:
Fe2+ → Fe3+ + e-
IONIC EQUATIONS INVOLVED:
Reduction Half: MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
Oxidation Half: 5Fe2+ → 5Fe3+ + 5e-
Overall Equation: MnO4- + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O
INDICATOR- KMnO4 acts as a self indicator.
END POINT- Colourless to light pink (KMnO4 in the burette)
1. Fill the burette with KMnO4 solution.
2. Pipette out 10ml. of Mohr’s salt solution into the conical flask.
3. Add half a test tube of dil. H2SO4.
4. Keep a glazed tile under the burette and place the conical flask on it.
5. Note down the initial reading of the burette.
6. Run down the KMnO4 solution into the conical flask drop wise with shaking.
7. Stop the titration when a permanent pink colour is obtained in the solution.
8. This is the end point. Note down the final burette reading.
9. Repeat the experiment until three concordant values are obtained.
OBSERVATION TABLE: (TO BE PUT UP ON THE BLANK SIDE USING A PENCIL)
Volume of Mohr’s salt solution taken =
Concordant Value = 8.8mL
CALCULATIONS: (TO BE PUT UP ON THE BLANK SIDE USING A PENCIL)
Calculation of amount of Mohr’s Salt to be weighed to prepare 100ml M/20 solution:
Molecular Mass of Mohr’s Salt = 392g/mole
1000 cm3 of 1M KMnO4 require 392g Mohr’s Salt.
250 cm3 of M/40 KMnO4 require =392/40g = 4.9g
N1M1V1 = N2M2V2
Where N1=5 (for KMnO4), V1=8.8mL , M1 =?
N2 =1 (for Mohr’s salt), V2 = 10ml, M2 = 1/20M
M1 = [1*(1/20)*10]/[5*8.8] = 1/88M = 0.01M
Strength = M X Molar Mass = 158 *( 1/88) = 1.79g/L
RESULT- (ON RULED SIDE)- The Molarity of KMnO4 = 0.01M
And the strength of KMnO4 = 1.79g/L